Also update to the tcell v2 PaletteColor api, which should keep the chosen
theme of the user intact.
Note, that if $TRUECOLOR is defined and a truecolor given, aerc will now stop
clipping the value to one of the theme colors.
Generally this is desired behaviour though.
This is a simple mostly straight forward switch to tcell in favor of
termbox.
It uses the tcell native api (not the compat layer) but does not make
use of most features.
Further changes should include moving to tcell's views.TextArea and the
general built in widget behaviour instead of the current ad hoc
implementation.
Regression: Cursor isn't shown in ex-line