added reorder pane command

you can reorder the groups with that command. My usecase for this:
I usually have two groups, one on top one on the bottom.
If i split the top one to left and right, the top right has the number 3 and not 2. With that command i can manually reorder it:
* go to group 3
* run reorder command
* type in 2
then i have the desired ordering. This is usefull for navigation via Ctrl+Number and for the View in the sidebar.

I can think of an other command which does this atomatically. This again could be performed after each command which creates or deletes a group.