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.