Use GtkSourceView search support

While working on replace all functionality, I have observed, that Scratch currently uses custom search and replace code.

Since 3.10, GtkSourceView has built-in support for search, replace, match count, replace all, and other goodies (regexp search, case sensitive/insensitive search) [1]. The current custom code could be migrated to be handled by GtkSourceView, resulting in less code duplication.
As Scratch already depends on Gtk+3.10, bumping the SourceView dependency to 3.10 shouldn't be a problem, as they should be available together on most systems.

[1] https://developer.gnome.org/gtksourceview/stable/GtkSourceSearchContext.html