Cycle-backward, fix restoring switcher popup

• FEAT: bind also switch-input-source-backward
  (fix #4.
  thanks to Yevhen Popok, @PotatoXPC)
• FIX: previously, disabling the extension and reinstating the switcher popup
  would brake repeated cycling, making it impossible to cycle further than
  the immediate next layout, not without first releasing keys and re-pressing them.
  • FIX: this could possibly also fix #5
    crashing when disabling the extensein (can't be sure, couldn't reproduce).
• refact: refetch sourceManager on each cycie-call, in case it has changed
  (instead of storing it in a global var on initialization).
• doc: comments describe new method's provenance
• doc: enhance README from user feedback & SO; mention similar extension by Osamu Aoki,
  better for multiple layouts.
• DOC: add LICENSE file AGPL.
• DOC: describe implementation provenance and challenges in comments.

support also gnome-shell 43

fix #3

support also gnome-shell 42

v5

feat: gnome-shell v42