Avoid wrong removal of objects when resizing the map

Since object coordinates are now in pixel coordinates, the visibility
check of the resize operation was wrongly determining most objects were
outside of the map boundaries and removed those objects. Now it should
correctly take the pixel coordinates into account. At the same time,
handling of tile objects and object rotation was added.

While working on this I found another bug however, which is that the
object positions are not adjusted correctly. This is because the
correction is still applied in tile coordinates.

Closes #617
Fixed position adjustment of objects when resizing map

The same issue still affects the "Offset Map" action, though.
Renamed resizelayer.cpp/h to match new class name
Fixed "Offset Map" action to offset objects properly

Objects were still getting offset in tile coordinates even though their
position and size are now in pixel coordinates.
tile layer: added horizontal and vertical offset properties
map reader: read horizontal and vertical offset for tile layers
map writer: write horizontal and vertical offset for tile layers
property browser: added horizontal and vertical offsets for tile layers
isometric renderer: first take at rendering offseted layers
map renderer: added method to calculate the max offsets from all the layers
isometric renderer: use the max layers offsets to correctly calculate the map size
map: moved the logic to calculate offsets to the map instead of the renderer
layermodel: recalculate map offset on layer offset change