maze: add sport shoes, indicating "run_fast"-mode

With this commit, tux puts on some red, fast running shoes, if
run_fast-mode is enabled. This will indicate wether run_fast-mode is
turned on or off.
maze: add fast_mode toggle button to board

This commit adds a button to the upper left corner of the screen.
Clicking this button enables/disables fast_mode.

It has the same functionality as clicking on tux.
maze: add fast_mode graphic files to Makefile.am
maze: disable run_fast controls in maze3D

The run_fast-mode is not in effect with the 3D maze. So we do not show
the toggle-button or the red shoes.
mazes: update manual to explain run-fast-mode

Added some text to the activity manuals, explaining the run-fast-mode
and its toggle-button.
The manual was extended in the three maze-activities, which support the
run-fast-mode. In the 3D maze, there is no run-fast-mode.
maze: fix Tux running beyond the door in fast-mode

In the previous implementation (from 2002), fast-mode was not
immediately disabled, when Tux reached the exit. Instead of that,
Tux kept running further along an unambiguous path, until he
reached a fork or a dead end, although he already reached the
door. => He ran through the door.

This looked very strange. Now this is fixed.
minor: maze: fix compiler warning

Since we removed the level-check as end condition for run-fast-mode and
replaced it by a check for gamewon, we do not need the "level" variable
any more. Nice of the compiler to point me to it. ;)
maze: fix buffer overflow, when playing long time

** Symptom **
While testing my run-fast patch, I encountered the following bug: When
you play maze activities for a long time, something in the internal
datastructure breaks.
1. the exit is printed at position y0, but jumps to other random
   positions y1, y2, y3, ... while playing
2. Tux starts running through walls as there were none, while he is
   block, where no walls are
3. Tux walks totally out of the maze (x or y position < 0)
4. GCompris exists completely, but without error message. It just
   disappears and you are back to console.

These effects start to appear, if you start GCompris, goto a maze
activity, switch to level 20 and play this level about seven times.

** Reason **
There is an array of fixed size, recording your "green" steps for each
level: position[]
The variable "ind" is used to indicate the next index in position[] to
write to.
This variable is increased while going through a level, but not reset at
the beginning of a new level. So playing many levels (doing many steps),
"ind" overflows position[]'s size of 740.
This leads to overriding other variables in memory, like the exit's
position, and finally crashing GCompris.

** Fix **
The "ind" variable is now reset to 0 at every level start (not only
at activity "load". So I could flawlessly finish level 20 20 times. ;)

This bug is kind of related to commit:
  f95663f2df7df2f3de8f98932b730cf49fed5a91
maze activities, reworked the home run button

Sadly even when scaling down an svg image we loose a lot
of quality. This patch makes the fast-mode-button smaller to
avoid having to scale it down in the code.
music activities, removed useless timers variables

In several places in music activities each call to a timeout_add
was saved in a list named self.timers. This list was not used anymore.

This patch removes these variables when not needed and add a protection
against callcack being called after the end of the activity.
music activities, protection against disabled fx

Now when the sound is disabled in GCompris the icon is
shown with a sound error and when entering an activity
it displays a dialog informing the user that the sounds
must be enabled.
explore activities, now their icons check audio

Added in explore activity a mandatory_sound_file to let
GCompris at runtime display a broken audio icon if the
sounds is disabled.
explore activities, added an 'audio' mode.

This mode is specified in the xml menu file. If given
the explore code will display a dialog asking the user to
enable the music.
piano composition, replaced text buttons by images

text buttons takes a lot of space in this activity
which is in need for real estate.

Icons comes from openclipart tango set.
piano composition, code reorganization

Moved all the connect() lines close to the creation
of the items. This makes the code easier to read and
maintain.
piano composition, changed lyrics layout

Changed the layout of the lyrics to have the title and country
displayed on the left. Now we have enough space for the lyrics.
piano composition, tooltip fixes.
piano composition, added a cute back to the bar

The lower button bar has now a cute background to highlight it.
piano composition, better buttons

Replaced the textual button "Erase and Change Clef" by
an icon based one.
Fixed missing files in the resource makefile.
piano composition, added background to lyrics

Now lyrics have a colored background.