Modified the movefocus dispatcher to prioritize focus change within groups.

[normaltaro]

Describe your PR, what does it fix/add?

It modifies the movefocus dispatcher logic to switch directional focus within groups first:

• Check if the current window is part of a Group.
• movefocus l/r can now be used to switch tabs within the Group
• If the current window is the first/last tab of its group OR movefocus u/d is pressed, then the usual directional focus logic applies.

In other words, a sideways directional focus change is prioritized within groups first.

How I think it benefits users?

• Better defaults
  • I think most users (like me) expect the directional flow of movefocus dispatcher to be followed within groups as it does in other window managers. I felt locked out of tabbed groups and never really used them because of this.
  • It is not possible to do so using changegroupactive along with movefocus as they require separate keybinds which do not conflict.
  • This keeps the last active index intact when focus is moved out of the group.
  • Personally, I think the overall experience with this patch is much better compared to other window managers regarding tabbed groups (like i3)
• Example:
  • mod h/j/k/l with movefocus l/r/u/d is intuitive for groups too.

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

I haven't found any bugs. It works as per the logic described above.

Is it ready for merging, or does it need work?

I have tried with all kinds of group orientations and it works good. It is a single block of 13 lines and could be merged as-is. :)