Devices: Add buttons to touchpad to match symbolic

[wpkelso]

Related: #881 , #1271

Adds left and right buttons to the touchpad to increase clarity out of context and to more closely match the symbolic version.

[danirabbit]

I don't love this change, especially since it's more common for touchpads on laptops to not have external buttons. I think it might be more worth looking at where/if this symbolic icon is even used (since this is outside of the fd.o spec) and maybe either dropping the symbolic or considering how it can be adjusted in context.

I think the color icon is only used in System Settings. It might be used in the bluetooth indicator, but I think bluetooth touchpads are considered by Bluez and Upower to be input-tablet devices

[wpkelso]

It looks like [Upower}(https://upower.freedesktop.org/docs/Device.html) does differentiate between tablet and touchpad in it's type property, but I don't know enough about Upower to know if that actually means anything.

Looking through all of the dialogs I could find on Horus, I didn't see any uses of the symbolic version anywhere, and no uses of the color version outside of system settings. Grepping my root didn't turn up anything outside of it being included in Adwaita (although thats probably not the best way to do it lol), nor did looking through the source code for bluez.

Given the ambiguity, I wonder if the better way is just to not touch either icon.