CompletionWords: enable code coverage
CompletionWords: add unit tests for library_find_first()

It is either broken, or I didn't understand what should return the
function.
CompletionWords: fix library_find_first()
CompletionWords: little clean-up and code simplification

Some new GLib functions simplifies the code.
CompletionWords: simplify the dispose()
CompletionWords: remove match()

It is the default value, so no need to override the function.
CompletionWords buffer: clarify idle_scan_regions()

The code was correct (I think), but it was hard to understand. Now it is
still correct (hopefully), but a little easier to understand.
CompletionWords library: simplify the code

Use the new g_sequence_lookup() function.