Rename tile selection functions in map document.

setTileSelection is now setSelectedArea
tileSelection is now selectedArea
tileSelectionChanged is now selectedAreaChanged
Renamed undo command ChangeTileSelection to ChangeSelectedArea.
Renamed changetileselection.h/cpp to changeselectedarea.h/cpp
Merge pull request #666 from despondentdonkey/refactor

Rename tile selection functions and undo command.