support key bindings that are sequences rather than chords

[wang-d]

Is your feature request related to a problem? Please describe.
I'm getting lots of conflicts with GNOME or other programs when I try to customize the keyboard shortcuts. Also, Super is the modifier with the fewest conflicts with other programs, but I find it kind of awkward to press. It's especially difficult to hold Super while pressing other keys with the same hand, as the default keybindings demand.

Describe the solution you'd like
I'd like the ability to bind things to sequences of keys instead of chorded keys. E.g., M-SPC h, meaning, hold Alt, tap Space, release both, tap h.

Other programs use this support to implement a UI idiom called leader keys where related operations are bound to key sequences sharing a common prefix. E.g., in stumpwm and ratpoison (two other tiling window managers) all window manager keybindings start with C-t (Ctrl-T) by default.

Leader keys are nice because I only have to find a single key chord not already claimed by another program. The following keys I can pick for maximum mnemonic value, without worrying about conflicts.

Also, once I type the leader key, the window manager knows I'm talking to it, and it can provide hinting. For example, it can pop up a list of possible next keys and their meanings. I don't think stumpwm has a name for this but in emacs this is called which-key mode. It makes complicated keymaps much more discoverable.

Describe alternatives you've considered
Tried using the default keybindings. Super-D is just very hard for me to type.

Tried being really careful about avoiding or resolving conflicts. That was hard.

Tried moving super on my keyboard, but I'd really like to keep my hands on home row.