WIP New "--uifont" option to specify UI font

If not specified, or "default" is specified (e.g., to override
a higher-up config file setting), it will fallback to what we
have #define'd in fonts.h as PANGO_DEFAULT_FONT, "DejaVu Sans".

It appears to fallback to this (or a reasonable fascimile) if
the font specified by the "uifont" option doesn't exist
(e.g., "tuxpaint --uifont=ABCD1234").

For https://sourceforge.net/p/tuxpaint/feature-requests/146/

src/tuxpaint-completion.bash

@@ -53,6 +53,7 @@ _tuxpaint()
                         --newcolorsfirst --newcolorslast \
                         --colorsrows=1 --colorsrows=2 \
                         --colorsrows=3 \
+			--uifont --uifont=default \
 			--sysfonts --nosysfonts \
 			--nostampcontrols --stampcontrols \
 			--nostamprotation --stamprotation \