add precompiled header.

Hi! i love tiled project.
but it compiled is too slow in windows. i add add precompiled header.
i don't know the pch file can work on other platform.
i add in `win32` section in tiled/tiled.pro
need to test on other platform.

also i add `QMAKE_PROJECT_NAME = libtiled`
because create vs project dll and exe same name.

Is it somehow necessary to include the Qt module name for these includes? I'm avoiding this everywhere in Tiled, which allows it to still compile against Qt 4 where these files were in `QtGui`.