Add shortcuts and buttons to navigate through search results
This merge request is a rebase of the last version of merge request !108 (closed) with some improvements taken from the @albfan proof of concept using a patched GTK+3.
The improvements taken are:
- Exposes a "show_buttons" in gitg-ext-searchable interface
- Adds default virtual implementation of "search_move" and "show_buttons" of same interface
Shortcuts available to search are:
- the default GTK+ 3 search shorctus: Ctrl+G / Ctrl+Shift+G
- these added shortcuts: F3 / Shift+F3
As written in the commit: due to use of the workaround, GTK+ 3 logs these warning messages on button click:
(gitg:2): Gdk-WARNING **: 23:24:41.192: Event with type 8 not holding a GdkDevice. It is most likely synthesized outside Gdk/GTK+
Fixes: #437 (closed) #60 (closed)