Adding groups as subgroups in large group trees - suggestion for improvement

[AEgit]

@koppor thought it might be good to create a separate issue for this suggestion (#9754 (comment)).

Scrolling within the groups tree allows one to add groups to other groups as subgroups. This works well with small or moderately sized groups but if you are dealing with thousands of groups, the scrolling speed is not high enough, to properly work with this approach.
On the other hand, further increasing the scrolling speed might also not be a solution, since you will not be able to precisely pinpoint the group you aim to be come the new parent group if the scrolling speed is too high (see also #9754).

I suggest the following two possible solutions (I would also be keen to hear from other people what they think would work best):

1. Have additional control on groups when using group filtering: When the groups filter is applied, groups that have already been selected prior to filtering are not removed but kept in the filtered groups panel. This way you could quickly add groups from one part of the tree to the other by selecting the relevant group and then filtering for the new parent group. So basically you select one or multiple groups. You then apply the groups filter - the previously selected groups and the filtered groups remain visible in the groups panel. You can then add the selected groups to the filtered ones as subgroups.
   Note, however, that this might get a bit tricky to implement (?) if you allow multiple groups to be selected prior to filtering.

2. Select the groups you want to add as new subgroups in the groups panel. Then filter or scroll to the group you want to use as parent group. While filtering or scrolling the previously selected groups remain selected (but are not necessarily visible). Once the group you want to use as parent group is visible, rightclick on the respective group. A menu opens that allows you to add the selected groups to the designated parent group.

(3. Finally, there would also be the option to just add another level in the scrolling speed - currently there are 3 levels. But, as mentioned above, that would probably not be a good approach to the issue.)