Removed unused rename icon
Avoid hang on Mac OS X when drawing certain ellipse objects

Trying to draw an ellipse with 0-width is causing a hang in
CoreGraphics when drawing the path requested by the
QCoreGraphicsPaintEngine. Draw them as rectangle instead.

In the isometric renderer, a rectangle is drawn as well anyway,
so the drawing of a 0-width ellipse is simply avoided.

Fixes #613
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
tilelayer: margins should be positive
changetilelayer: merged both offset commands into one to fix a bug when setting the offsets
maprenderer: fixed implementation of screenToTile and implemented tileToScreen using offset
isometricrenderer: correctly draw the offseted tile layers
maprenderer: screenToTile and tileToScreen accept Layer instead of TileLayer
maprenderer: the grid is drawn in an offset according to a given layer. Implemented only for isometric.
isometricrenderer: include drawing margins in the map size
maprenderer: always apply the mapOffset on screen/tile conversions, even if it's not a tile layer
maprenderer: just realized the horizontal offset was reversed