herbstluftwm: Update to 0.9.1

Release 0.9.1 on 2020-12-28
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  * Client minimization (controlled by the attribute 'minimized' of every client)
  * The 'index' attribute of tags is now writable. This allows adjusting the
    order of existing tags.
  * New child object 'focused_client' for each tag object.
  * New child object 'focused_frame' for the tiling object of each tag object.
  * New child object 'parent_frame' for each client providing the frame the client sits in
  * New command 'mirror'
  * New command 'apply_tmp_rule'
  * The 'apply_rules' command now reports parse errors
  * Reserve space for panels that do not set _NET_WM_STRUT e.g. conky windows
    of type 'dock'.
  * Only build json object doc if WITH_DOCUMENTATION is activated
  * Bug fixes:
    - When hiding windows, correctly set their WM_STATE to IconicState (we set
      it to Withdrawn state before, which means "unmanaged" and thus is wrong).
      This may require restarting pagers when upgrading hlwm live.
    - Fix crash in 'split explode'
    - 'split explode' preserves the window focus (as in v0.7.2 and before)


(nia)