Added Screen coordinates, taking the role of the old Pixel coordinates.
Map Objects are now in Pixel units internally.
Corrected IsometicRenderer's pixelToScreen/screenToPixel.
Merge branch 'screencoords' into mapobjectunitsv2
Updated object tools to use pixel units as needed.
Removed unnecessary conversions in IsometricRenderer.
Merge pull request #591 from mauvecow/screencoords

Renamed "pixel" to "screen" coordinates and introduced a new "pixel" coordinate type.
Merge pull request #592 from mauvecow/mapobjectunitsv2

Map Objects' internal representation converted to pixel units
Fixed snapping to grid when moving and pasting objects

When pasting objects it was dealing with pixel coordinates rather than
tile coordinates for aligning.

For the moving, the alignment position is now in pixel coordinates but
was still treated as being in tile coordinates.
Removed unused TilesetModel::mAllowDragging

In fact this property was not needed since the view already has an
option for this.

Coverity-issue: 1165683 (Uninitialized scalar field)