Implement module search.

This works by wrapping the ListStore of every SectionFrame inside a new TreeModelFilter.
Make the SceneManager go back when searching from a loaded module.
Focus the searchbox on launch.
Enforce the section order.

This is done by prioritizing the SECTIONS tuple above the other eventual sections
found by looking through the modules' desktop files.
Added panel (tint2) configuration module.

This is basically a port of tint2-panel-config.
SectionFrame: Sort modules by name.
Merge branch 'master' into debian
0.29-1