Add button glyph font files

They are not used yet in this commit - this just adds the graphics and
data for the glyphs. It also adds a <fallback> tag to font.fontmeta to
use buttons_8x8.

The icons themselves are made by Reese Rivers - see #859.

desktop_version/fonts/font.fontmeta

@@ -68,4 +68,5 @@
     <special>
         <range start="0x00" end="0x1F" advance="6"/>
     </special>
+    <fallback>buttons_8x8</fallback>
 </font_metadata>