Cycle tabs with Ctrl+Tab

[jeremypw]

Fixes #834

Rather than use the native Hdy.TabView select_next_page () and select_previous_page () functions, which do not cycle, a custom function is used.

[ryonakano]

If we remove the unnecessary cast against n_pages here, we can remove the following places too , excepting the cast against the result of focus.clamp () :

[ryo@b760m ~/work/tmp/terminal (jeremypw/cycle-tabs =)]$ git grep -n n_pages | grep -F '(int)'
src/MainWindow.vala:485:                        notebook.selected_page = notebook.tab_view.get_nth_page ((int)notebook.n_pages - 1);
src/MainWindow.vala:718:                var tab = notebook.tab_view.get_nth_page ((int)(focus.clamp (0, notebook.n_pages - 1)));
src/MainWindow.vala:727:            int pos = (int)notebook.n_pages
[ryo@b760m ~/work/tmp/terminal (jeremypw/cycle-tabs =)]$

[jeremypw]

Nice use of git grep - I was not familiar with that.

[ryonakano]

LGTM and works as expected. Thank you for your fix!