Plugin manager

With a plugin manager we only search and load plugins once and create
models for categories based on the list of plugins we found.

We also change the plugin API and move metadata to desktop entries that
will also be used to run the preflets from the desktop environment.

Closes #24
app: Load preflet passed to command line

Closes #27
Say hello to the keyboard panel

Initial keyboard configuration, shortcuts are not yet implemented.

Closes #23