AppMenu: Fix regression and another issue

[jeremypw]

Changing to temporary settings object introduced a regression as they have signal handlers attached. The default button label did not update when the zoom change buttons were used.

• Persist settings as static objects

It was also noticed that the zoom buttons were not correct if the menu was re-opened after switch to a different tab with a different viewmode/zoom-level. This bug was found to be pre-existing but was fixed here anyway.

• Set up the buttons every time the popup menu is opened.

It was noticed that it is not now necessary for the toplevel to update the view-mode even if the menu is not open and the relevant code was moved inside the AppMenu. This is pretty much the same as #2731

[jeremypw]

Closing as this now only fixes a very corner issue - the dateformat buttons updating when the setting is changed externally (e.g. with dconf-editor or another Files Window) and the AppMenu does not play well with multiple windows anyway.