Tell the compositor when the shell is ready

Call desktop_ready request when all shell windows are visible.

Issue: #38
Default font name and size

A default font name and size were not set, causing the desktop to use
the first font found.

This was evident installing fonts for different languages such as
ttf-indic-otf for the Malayalam language used in India.

This fixes issue #39.

Issue: #39
Use "Droid Sans" as default font

"DejaVu Sans" was not a bad choice but this looks better to me.