dh-preferences: Expand tabs

https://bugzilla.gnome.org/show_bug.cgi?id=730445
window: Use view-context-menu-symbolic for the gear menu

This is a new, dedicated icon name for this purpose.
See https://wiki.gnome.org/Initiatives/GnomeGoals/GearIcons

Bump required GTK+ version for this.

https://bugzilla.gnome.org/show_bug.cgi?id=732897
add F9 and gear menu action to toggle side pane

https://bugzilla.gnome.org/show_bug.cgi?id=729512
ui: center tab labels

https://bugzilla.gnome.org/show_bug.cgi?id=724098