Save the stagger axis also for staggered isometric maps

Fixes #839
Lua plugin: Added staggered and hexagonal map related properties
Added color properties to QtPropertyBrowser framework.
selectedTilesChanged signal added to MapDocument.

updateCustomProperties when selected tiles and map objects change.
Added updatePropertyColor in PropertyBrowser.

If selected object does not have property gray out name and value.
If selected object has different value then gray out value.
Update property colors when properties change.

Add properties to browser if current object does not have them.
Merge pull request #779 from parkm/propertyVisuals

Visual feedback for multiple object properties
Silence MinGW warning

libtiled/tilelayer.h:320:20: warning:
'const Tiled::Cell& Tiled::TileLayer::cellAt(int, int) const'
redeclared without dllimport attribute after being referenced with
dll linkage
Updated libtiled Qbs project
Added Qbs files for Tiled plugins

All plugins except the Python plugin for now, which will need a probe.
Qbs: Enabled Python plugin with PkgConfigProbe

Unfortunately, this won't work on Windows and likely not on OS X either.
Qbs: Fixed building of Python plugin on Windows

While hopefully not breaking it on Linux.
Qbs: Fixed Python plugin compilation on Linux

Apparently the probe can't be inside of the Properties.