keybinding: Store flags in MetaKeyGrab

The external grab handler is shared across all external bindings and external
bindings have now different binding flags. For this reason, when rebuilding the
binding table there could be loss of information if we assign the bindings flags
of the external handler to all external bindings. Let's store the bindings flags
in MetaKeyGrab too and use this when rebuilding the binding table to avoid the
above issue.

GNOME/mutter!169
3 jobs for !169 with fix-issue-gsd-68 in 7 minutes and 25 seconds
latest detached
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  Review
passed #222001
check-commit-log

00:01:49

 
  Build
passed #222002
build-mutter

00:02:30

 
  Test
passed #222003
test-mutter

00:03:05