Fixed object drawing order for image saving and mini-map

Currently objects are always drawn top to bottom. For the regular map
view this is taken care of by setting the 'z' property of the items
to their 'y' coordinate. However, when saving as image or when rendering
the mini-map, the objects need to be sorted manually.
Fix library directory order

This moves path to -ltiled before system library locations, fixing the
case where libtiled from (older) installed version of tiled is
picked up instead of newly built one.
Ignore mac build artifacts
Fixed some plurals in English translation

The actions on polygon points were not translated yet. These are needed
for proper plural handling.
Fixed line widths when zooming in Qt 5

The grid and selection lines are not supposed to be affected by the zoom
level. In Qt 5, the default QPen width changed from 0 (which is
independent of the current transformation) to 1.
Merge branch '0.9'
Improved interface for rotating objects

The logic was reimplemented in ObjectSelectionTool. This allowed for
rotating objects around their center as well as rotating multiple
objects together.

Sponsored-by: Ben Wales
Rotate objects around their center instead of the bottom left

This makes the implementation a little bit more complicated, but it
should be more intuitive overall.

This change is mostly about changing the interpretation of the
'rotation' attribute and is not backwards-compatible. However, rotation
support was added recently so I don't expect anybody to depend on the
previous behavior. Eventually the transformation origin could also be
made configurable, if there is a need for that.
Implemented moving objects with arrow keys

By default the selected objects move by pixel. The Shift modifier speeds
it up to move by tile (or by subtile when fine grid snapping is
enabled). The Control modifier causes the event to be ignored, so that
it can be used to move the view.

This behavior is similar to that seen in Inkscape and GIMP.

Closes #342

Sponsored-by: Ben Wales
Don't handle keys while moving or rotating with the mouse

The undo stack gets confused when that happens.
Unduplicated the duplicate and remove objects actions

Also shared the implementation of moveObjectsToGroup by moving that
function into MapDocument.
Fixed the absolute paths in the desktop file

The application icons now install into the system icon theme. This also
results in the system always picking the best available icon to display.

To make installing simpler (and to make the names more consistent) the
icon files were moved into the folders of their resolution.

Fixes #383
Fixed the tab order in the Object Properties dialog
Small arbitrary cleanups in AutomappingManager
Update JSON plugin to support JavaScript (JSONP) load/save

Closes #374
Small cleanup
Made Tiled registering *.tmx as MIME-type

Tiled now registers the MIME-type application/x-tiled based on the file suffix
*.tmx. It also registers itself as application being able to handle this
MIME-type. This means systems following the freedesktop standard [1] will now
display tmx files with a tiled icon and open those files with tiled.

[1] http://standards.freedesktop.org/shared-mime-info-spec/shared-mime-info-spec-latest.html
Flare export plugin: Remove checks for certain layers being there

The flare engine evolved and supports 2 more layers optionally.
The three layers which were checked being there, should still be there,
but are not necessary. Also I cannot recall any errors due to missing
or miss spelled layers during flare development, so I think we do not
need this check for flare.

The check for these three layers unintentionally hindered other projects
using this format, which is another reason to drop the check.