Fix a few potential out of bounds by placing proper checks, which should
be relevant if all tabs are removed for some reason.
Also avoid iterating all tabs in the invalidate handler, since we are
only interested in whether it's the selected tab either way
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
