ui: Refactor input buffer handling

All input buffer code was moved to os/input.c, and `inbuf` is now a `RBuffer`
instance(which abstracts static buffer manipulation).
Merge PR #1300 'Refactor input buffer'
main: extract normal_redraw out of main_loop
edit: remove static modifier from ins_redraw
events: Remove K_EVENT special key handling
input: handle redraw-related updates directly in input_poll