tree/view: Make foreign-toplevel app_id fallback to class

It is not a part of the foreign-toplevel-management protocol to get the
class of a toplevel, only for getting the app_id.
For xwayland clients this is an issue because that means that you cannot
identify what application the toplevel refers to which is the point of
the app_id property.

By falling back to class when an app_id does not exist solves this problem.

Phoc also uses app_id and class interchangeably in their implementation
of foreign-toplevel-management, in fact they always do that and not only
for just this protocol.
https://source.puri.sm/Librem5/phoc/-/blob/c8d8a4c5440a6c1647b09dbd3bba7999f9cd433c/src/xwayland.c#L236

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