Detailed template information in UiTemplateSelector.

 * Add accels to UiTemplateSelector: "i" to show info and "Esc" to go back to
   template list.
 * Add Vala version information to template (optional).
 * Add long description to template (optional). Long description supports
   Pango markdown formatting.

 * Hide build output progressbar at the beginning (fixes dirty workarround).
 * Wrap long lines and add some comments.