[feature request] ability to modify tags directly, instead of modifing each credential

[HolgerHees]

In my scenario I have ~400 passwords. For that reason, from time to time it make sense to reorganize my tags, because a new classification fit better or I realize that a previous classification was not a good choice. Currently I have to change really a lot of credentials, which is quit heavy and annoying work.

BTW, currently, I migrate from keeweb to passman, and I take this step as a chance to rethink my tags. But I reorganize it still in keeweb because it is so much easier. After that I convert the kdbx file with a small python script to the passman json backup file format. To get this working properly I fixed already a tiny import bug. #781