Change island selection from being it's own selection mode,
along side vertex/edge & face modes, to a separate toggle.
This makes it possible to use sync-select without loosing functionality
as previously island selection didn't work when sync-select was enabled.
Now island selection supports sync-select although at the moment this
only works in face-select mode, when !138197 is merged this limitation
will be removed.
In practice this change is subtle, making it possible to toggle island
selection no matter which selection mode is currently active.
Details:
- The UI is mostly unchanged, this option is available
in the same location in the header & menu as a toggle.
- The shortcut 4-key is kept, toggling instead of switching modes.
- Enabling island selection no longer isolates islands,
it just acts on selection actions made after being enabled.
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