applets/taskmanager: two more fixes to thumbnail-less and/or vertical multi-window tooltips

The task tooltips when shown in vertical orientation (e.g. side panel) have quite a bit of wasted space: the App name for all windows in a group except the first, and empty window titles resulting from a matching app name or a media player. This makes the tooltips larger and more obtrusive than they need to be.

This change removes those lines instead of leaving them empty. This applies only to vertical tooltips, as all windows in a group should have the same height.

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For grouped tasks, the tooltip instance for a particular window can be completely empty (save for the close button) under some very narrow circumstances, namely the app name and window title have to be identical, the window has to be the second or later, there has to be no applicable subtitle (like virtual desktop number) and thumbnails have to be disabled. In those cases, there is nothing really useful to show, but having absolutely nothing seems even more broken.

This change makes the window title show even if it is a duplicate of the app name, but only if the tooltip entry for that window would otherwise be completely empty. As the conditions for what to show and when are getting hard to read, they were also refactored to make them more descriptive.

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Screenshots:

	before	after
empty lines
completely empty entry

empty entries with tooltips are unchanged; these look a little weird as well but it's not clear to me that putting the window title there is an improvement:

before and after	alternative

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