Previously, it was not possible to define keybindings with a modifier
and a function key. The `Key` enum only supported combinging the Ctrl
and Alt modifiers with letters. This is somewhat limiting: I would like
to make the Zellij keybindings more "distant" than those used in the
programs I use from within Zelilj, so that Zellij does not intefere with
those programs. Thus I would like to move some of the keybindings from
Ctrl+<character> to Ctrl+<function key>.
This change adds:
* support for function keys with the Ctrl and Alt modifiers in the
`Key` enum,
* support for parsing such keybindings from the configuration file,
* support for such keybindings in the protobuf which communicates the
keybindings to plugins, and
* support for these keybindings in the plugin API.
This is tested by modifying one of the e2e tests to include an example
of such keybindings. This verifies that the configuration is correctly
parsed, communicated with the plugin, and rendered.
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