Port cairo-xtext from Conspire.

This pull request replaces the XChat XText widget with the version I rewrote using pango and cairo.

It is many orders of magnitude faster than the old XText widget and due to using pango and cairo, can take direct advantage of hardware acceleration for drawing glyphs, such as XRender on X11 and DirectWrite on Windows.

This also introduces real transparency using compositing instead of the broken and slow pseudo-transparency in the old XText.