Graphics update for progressbar
Background image for progress bar. Fixes #1650

And somewhat minor refactoring. Now exposes attributes to xml.
Changed behavior which can be extended/generalized later:
* Fixed background size of currently 100px; has to match max amount
* Fixed progress tile of currently 1px
* No special left/right framing tiles
Use background attribute for ProgressBar
Call tile-laying even if no background exists

Turns out it is not important to draw the background behind other stuff.
int() for progressbar amount

Boat builder for instance was passing floats here.
"name" instead of 'name' for xml file

In the name of coherence.
Boat builder layout update

The funny max_size there restricts the bar height and thus scales all
icons, which is easier interface-wise than implementing own "height"
attributes and also more flexible even if weird-looking at first.
Clean up active_tab implementation for SelectableComponent (#1941)

Now specifies the desired tab's name instead of 0-based index. Sadly,
this needs a bit more magic to work because the tab index can only be
determined after checking which tabs would currently be displayed.
Add hint about where our new dev MP server lives
Merge pull request #2007 from unknown-horizons/progressbar

Add background feature to progress bar and relayout it

This is a first iteration but should still work as-is.
In particular it'd be nice to have a horizontal resize feature instead
of hardcoding the exactly 100 tiles as widget size.
Bar height can be worked around using a max_size height component while
not restricting width.
Fix progress bars staying at initial progress sometimes

In particular, when directly contained in a widget without auto-resize.
Scale progress bar width with max_size parameter
Prevent infinite recursion in pychan

No available max_size does not go well with relying on a max_size for
defining background image size.
progressbar: `fill` attribute for xml definition

Also roll back ProgressBar to one resizable image instead of tiling:
* There were too many images with a TilingBG of 1px tiles
* Stretching 1px tiles via pychan yields the exact same result as tiling

Might add a TilingProgressBar sometime later if we decide it's useful.
TilingProgressBar

What a mess. Refs #1650.
Currently unused but i guess we can find use cases for this and it's not
completely unmaintainable either. Will count size of left and right bar
frames towards the progress displayed (i.e. the max_size).
Usage:
<TilingProgressBar background="bg.png"
left="left.png" fill="fill.png" right="right.png" />
os.path.join
Fix #1740 - Font change on runtime

Don't ask me what this is supposed to indicate about pychan's font API.
Explicitly set metaclass for very custom widgets
Revert "Explicitly set metaclass for very custom widgets"

This reverts commit 812688405da55b2b2433e001f6ebf6916d6ba825.
Use manual inheritance initialization because super linearization
results in a order that results in problems with the __metaclass__.