Borders: Fix borders on active combat change

A scene can have multiple combat encounters, but only one
can be active at a time.
Changing from one to the other does not trigger any of the regular
combat hooks, but it can be detected using a combat /document/ hook.

Register on the combat document update hook and update the borders
when the active state of the encounter is changed.

Both the previous and new active combats produce an update, and
the previous combat becoming inactive always comes first.
This means that we can safely set all token borders from the
previous document to the default border and they will be
updated accordingly by the new combat becoming active.

Update the comment of the data member for
`combat_hook_update_token_borders()`, as we re-use it for
the combat switch and we didn't use the `turn` member anymore.

README.md

@@ -36,14 +36,6 @@ The module supports going back and forth in the combat rounds, as well as going
 - An image to use for the default/idle border
 - An image to use for the took turn/played/inactive border
 
-### Limitations
-
-There are currently one minor issue that might be fixed :
-1. The token borders will be incorrect when switching between different encounters in the same scene
-   - Producing a combat event or switching away and back to the scene will fix it.
-   - It doesn't appear that there is an event on combat switch that could be hooked into, which makes fixing the
-     issue uncertain.
-
 ## Token UI adjustments
 
 Given that we add a border *on* the tokens, it conflicts with the base attribute bars which are drawn over the token's