Bug 1102220 - Always place apps in the last group when installing. r=kgrandon,crdlc

If the last group is collapsed, instead of placing newly installed apps
into a new group, expand that group and place it there as normal.
Merge pull request #26334 from Cwiiis/bug1102220-install-in-last-group

Bug 1102220 - Always place apps in the last group when installing. r=kgrandon,crdlc