Since there is only one LED panel per terminal, redrawing for each session
is wasteful.
Furthermore, since one terminal can have many sessions (i.e. tabs), and
since the last session in the list might not be the current session, the
wrong LEDs might be drawn.
An easy way to demonstrate the bug is to enable ui.clock.enable, so that
the panel is redrawn every 100ms, and then to select a text field and enter
insert mode. Unless the current tab is the last tab, the insert-mode LED
will only briefly show that insert mode is enabled.
