xterm-color-chooser: put altscreen init code in a function
xterm-color-chooser: add true-color mode
xterm-color-chooser: downscale RGB bars
xterm-color-chooser: change "dark" key to 'k'
xterm-color-chooser: unify g/G code
xterm-color-chooser: fix system-color mode
Merge branch 'xterm-truecolor'

Implement true-color (RGB) mode in xterm-color-chooser.