Completion: split the code into sections

The sections:
- Utilities functions
- Show, hide and activate
- Info window
- Switch between providers
- Accelerators to activate the first proposals
- Various callbacks
- Handle population
- Properties, construction and destruction
- Public or non-static functions

A section is displayed like:

/**********
 * Section
 **********/
Completion: remove debug messages

I personaly use gdb, or I sometimes add temporary g_message()s at
very specific places. Those specific places change for each bug. IMHO
it's not useful to keep the debug messages.
Completion: fix the update of the window position

This bug was visible with test-completion. The completion window was not
placed correctly.
Completion: avoid memory leaks
Finalize the default instance of the StyleSchemeManager