Allow extensions to activate by trigger

As discussed, it would make the user experience smoother if extensions were more naturally integrated, so you don't have to search for them by hand or even by name.

The advertisement filter (adblock) and cookie manager could show up in the privacy preferences as buttons, which activate the respective extensions. Before that, they don't make for any memory or performance overhead.

The toolbar and hotkey editors could show up in the toolbar right-click menu.

The history list extension could trigger an infobar that appears the first time you hit Ctrl+Tab and offers you to switch tabs in the order you open and close them.

These extensions might be omitted from the extension list, to limit it to specialized extensions, which in turn makes it easier to go through.

API proposal by example (in Vala for brevity):
extension.add_trigger (MIDORI_TRIGGER_PRIVACY_PREFERENCES, _("Configure Advertisement filters"));
extension.add_trigger (MIDORI_TRIGGER_TOOLBAR_CONTEXT_MENU, _("Customize toolbar..."));
extension.resident = false; automatically load extension, omit from the list, unload automatically
extension.connect("triggered",...)