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:

1. Exposes a "show_buttons" in gitg-ext-searchable interface
2. Adds default virtual implementation of "search_move" and "show_buttons" of same interface

Shortcuts available to search are:

1. the default GTK+ 3 search shorctus: Ctrl+G / Ctrl+Shift+G
2. 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)