Allow selection of multiple groups

[tobiasdiez]

It was previously possible to select more then one group.

Related group settings:

• "Intersection",
• "Union",
• "Inverted",

[AEgit]

I'm using this feature quite often, so I would heavily be in favour to it being readded.

[lenhard]

This is implemented in #2845

[AEgit]

I can confirm that this works in:
JabRef 4.0.0-dev--snapshot--2017-05-18--master--018173ebd

I do notice, however, that the group selection is somewhat slower compared to 3.8.2 - every time I select a group in the groups panel, it takes ~1-2 sec to show the group entries. In 3.8.2 this selection happened in an instant. This might be related to #2837 or #2588

[lenhard]

Too bad. At least now much of the duplicated code is gone and I was hoping for a better performance. We should try to improve this some more before a final release.

[AEgit]

I might have to take it back. Today the performance issue I reported here, does not seem to appear. The group entries are shown immediately, when a group is selected. The only performance issue that remains is the one reported here:
#2588 (comment)

BTW: Can someone tell me, what the green colour around the item number reported next to each group in the groups panel means? Does it mean, that a selected entry is part of the group indicated in green? If so, I might have found another bug.

[tobiasdiez]

@AEgit Yes, green = all selected entries are contained in this group, brownish green = at least one selected entry belongs to this group.

[AEgit]

@tobiasdiez : Thank you very much - I think I found the bug, I mentioned (it just took me a while to find out, how to replicate it): see #2857