Fixed object drawing order for image saving and mini-map

Currently objects are always drawn top to bottom. For the regular map
view this is taken care of by setting the 'z' property of the items
to their 'y' coordinate. However, when saving as image or when rendering
the mini-map, the objects need to be sorted manually.
Fix library directory order

This moves path to -ltiled before system library locations, fixing the
case where libtiled from (older) installed version of tiled is
picked up instead of newly built one.
Ignore mac build artifacts
Fixed some plurals in English translation

The actions on polygon points were not translated yet. These are needed
for proper plural handling.
Fixed line widths when zooming in Qt 5

The grid and selection lines are not supposed to be affected by the zoom
level. In Qt 5, the default QPen width changed from 0 (which is
independent of the current transformation) to 1.
Merge branch '0.9'
Improved interface for rotating objects

The logic was reimplemented in ObjectSelectionTool. This allowed for
rotating objects around their center as well as rotating multiple
objects together.

Sponsored-by: Ben Wales