Port to GTK+ 3 while maintaining compatibility with GTK+ 2.
Change the color of the "current tile" cursor based on ToolTest results

If the current tool can't be used in the current location, the rectangle
displayed under the cursor turns red instead of blue.
Disable GTK3 by default since it causes a memory leak
GLTileEngine: abstract EGL stuff into a separate EGLPlatform class

This paves the way for an eventual GLX backend.