Resolve section aliases on the fly

The original code resolved them when the menu was requested as I thought that made constructing the dbus menu
easier as in dbus menu it's just a contiguous list of actions. However, this also changed the mapping from
dbus action to GTK action (the latter does not have a unique ID system), so when a GTK app signalled a change
(e.g. a menu item got added/removed), the signal was for the original position of the item and not the
resolved one we were using internally.
By resolving everything on the fly, the IDs stay correct, and updating the menu mostly works now.
There's still something funky going on when moving from e.g. LibreOffice splash to LibreOffice Writer
but that needs to be investigated now.