Add option to reorganise groups

[merv1n34k]

Hey!

First of all, thank you for this amazing project!

Issue

In the current version you have an option to move sessions across groups, but you can't do that with groups. Ex: move subgroup project1 and all its sessions from group personal to group work

Proposal

To avoid confusion with moving sessions, reorganisation of groups can be called change.

GUI

GUI already has M key to move sessions across groups, we can use the same UI and logic to reorganise groups, e.g:

Press M -> Open dialog "Move group to" -> Select destination from the groups list-> Press Enter -> "Group is reorganised"

CLI

For CLI I suggest adding new subcommand agent-deck group change <name> [<dest>], where <dest> is an optional parameter which should be group name, subgroups should be expressed with full path, i.e. group/mysubgroup/subsubgroup to avoid ambiguity, as we don't have groups IDs. Leaving second option empty will move group to root, if no name collision happens.

Alternatively, we can rename "Move session to group" to group change and call it Change group for session, and keep group move for group moving functionality. IMO this is more clean approach.

I think the proposal is not too hard and counts as good first issue, so I can implement this myself. What do you think?