This adds support for specifying a binding for a specific group. Any
binding without a group listed will be available in all groups. The
priority for matching bindings is as follows: input device, group, and
locked state.
For full compatibility with i3, this also adds Mode_switch as an alias
for Group2. Since i3 only supports this for backwards compatibility
with older versions of i3, it is implemented here, but not documented.
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