Binding modes/binding submaps

The ability to configure multiple binding modes with separate binds.

An example use-case is a "launcher" mode, where e.g. you press Super+D, then instead of the normal binds, you have binds to run applications, like F for firefox, N for nautilus, T for terminal, etc. So you press Super+D+F to run firefox.

There's a WIP implementation for binding modes: https://github.com/YaLTeR/niri/pull/532. The author's GitHub account had disappeared, but I remember about the PR and will get to it at some point.