After 1e799dd26e, the logic to recognize a temporary File Browser window
didn't work correctly anymore. It would recognize a maximized File Browser
inside a temporary window as a temporary File Browser window, and attempt to
close this (rather than returning to the previous layout).
The logic there was pretty weak, and still is I think. A more stable solution
would need bigger refactoring.
With this, it's also not possible to maximize or fullscreen an area within a
temporary window (Preferences, File Browser, render window) anymore. Think that
won't make a noticable difference, since you couldn't open multiple areas there
anyway, and the area seems to be maximized already.
Cleaned up the code a bit to not become more confusing with the changes.
78d2ce19c4