Implement right-click popover menu

Add the possibility to handle mouse right-click using a GtkPopover to show
a menu with buttons for copying/pasting text and, when hovering an URL,
also opening/copying the URL. Note that GtkPopover requires GTK+ 3.12, so
the feature is disabled by default.
Disable usage of header bars by default

Headers bars need GTK+ 3.10, so make the option disabled by default.