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
Renamed the tmx MIME type to application/x-tiled-tmx

This should make it easier to add further MIME types of plugins.

The icon is not renamed to application-x-tiled-tmx in order to allow
other plugins to depend on it in future too.
Small tweaks to the desktop file

The TryExec line makes sure that tiled is only listed if the executable
exists. Meaning that the desktopfile should not be displayed as an menu
entry if the executable is missing in $PATH (for example if you install
with a custom prefix).

Setting the GenericName might have some use for some window managers /
application menus (I was unable to find this displayed anywhere in the
gnome-panel manu, but it cannot hurt).
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.
Merge github.com:bjorn/tiled