Convert PidginStatusBox to a GtkMenuButton

This moves to a menu system to populate the list of statuses, and the new `PidginStatusDisplay` to show them. I tried a `GtkDropDown` previously, but that was unable to show the separators that we previously had. Consequently, it uses actions a bit more then before.

However, since menus don't support icons+text at the same time, and doesn't have a factory property, we do need to manually create custom widgets a bit, but it's mostly only a button + the `PidginStatusDisplay`.

The main display is now centred, but there seems to be nothing we can do about that, as the box with that alignment is internal to the `GtkMenuButton`. If we don't want that, we could switch to managing a `GtkToggleButton` and `GtkPopoverMenu` ourselves.

Testing Done:
Opened status box and confirmed that all primitives and saved statuses were there as expected.
Modified a status, and confirmed that the menu updated to match.
Added/deleted statuses from the manager and confirmed they menu updated to match.
Chose the New Status/Saved Statuses menu items and confirmed they opened the correct dialogs.
Picked a bunch of statuses from the menu items and confirmed the main display updated, and some updates were sent out to protocols (e.g. Demo disconnecting when offline, etc.)

Reviewed at https://reviews.imfreedom.org/r/2386/