Fixed compile errors with GCC 4.8

Also fixed some warnings from header files that were being particularly spammy to the compile output.

Just asking you to accept these so that I won't have to fix them again next time I check out "TileZed". :-)

Ever planning to open some more pull requests to Tiled, btw? You've done an amazing amount of work which makes it a bit hard for me to know whether there's anything of interest to general Tiled users out there. Except I can just check it out and see if there's any features I like. But, I guess we've been diverging quite a bit now making such things ever harder.