[Feature Request] Option to enable libinput "Emulate middle mouse button"

[dmccombs]

Is your feature request related to a problem? Please describe.
Coming from Xorg, I'm very used to the feature of emulating a middle mouse click by pressing the left and right click buttons together for pasting.

Describe the solution you'd like
Add an option to mouse settings to enable this Wayland feature in Cosmic:
https://wayland.freedesktop.org/libinput/doc/1.11.3/group__config.html#ga584d862a116fb0a26cc95304f8c13eb1

This should exist separately for touchpads and mice.

Describe alternatives you've considered
I've looked around for enabling this in Wayland via a separate utility/command, but haven't found anything.

[dmccombs]

This is already supported by cosmic-comp to be set, it's just not in cosmic-settings or the UI. Creating a ~/.config/cosmic/com.system76.CosmicComp/v1/input_default file with the following contents and logging out/in successfully enables middle mouse emulation on my mouse:

(
    state: Enabled,
    left_handed: Some(false),
    middle_button_emulation: Some(true),
)

I'll take a look at adding the config to cosmic-settings and the UI at some point soon and open a PR.