view: implement GtkBuildable::get_internal_child() for "completion"

To be able to configure the completion directly in the .ui file, like it
is done in test-completion.

https://bugzilla.gnome.org/show_bug.cgi?id=736367
completion: implement GtkBuildable::get_internal_child()

For the info_window.
CompletionInfo: set a border width of 3

So that the label or another widget has some space around it in the
calltip.

Since the main completion window is also a CompletionInfo, set the
border width to 0 for it.
[l10n] Update Catalan translation
[l10n] Updated Catalan (Valencian) translation
Updated Hungarian translation
Updated Slovenian translation
Add warning comment about big buffers present in a Private struct
Don't always invalidate the whole gutter

Instead, invalidate the whole lines gutter renderer only when needed.

WARNING: during file loading the gutter is redrawn a lot. There is a cycle:
redraw gutter -> invalidate gutter -> load chunk of text -> draw gutter ->
invalidate gutter -> etc.
Instead of taking the total line count of the buffer, a solution is to compare
the last visible line number, but it's slower.

This should prevent degenerate cases where the gutter window is being
continuously invalidated during certain operations, for example search.

This is needed to have decent search performances on Mac OS X (but this
is not the root cause why the search was really slow on OS X, it is
probably because drawing is much slower than on Linux).
test gutter perfs