Revise carvac_dv keymap for easier alt-tab (#1871)

Revise carvac_dv keymap for easier alt-tab (#1871)

* Add carvac_dv keymap for mitosis

* Add mouse keys

* move backspace, etc, and fix tab

* remove commented-out functions in keymap

* Fix scroll buttons and add left/right scrolling

* Make num momentary, add comments, and clean up

* fix mouse scroll acceleration

* Add tab, remove bksp, move print screen

Having tab next to control and alt makes for much easier
alt-tabbing and ctrl-tabbing.

That displaced print screen, but I had never used the non-layer
backspace on the right hand, so I moved printscreen over there.