Use TabNew/TabClosed for the "autoshow" event when tabline is used only
for tabs (show_buffers==0). This fixes case of visible tabline when
tab_min_count == 2 and user executes ':tabonly' command - the tabline
should disappear but there is no TabEnter event triggered in this case.
Closes#2389
This is an experimental feature that allows to display the statusline in
the tabline. It might still be a bit rough, but seems to work so far.
Remaining problem:
- Mode changes are not immediately detected, only after moving the
cursor
fixes#1388closes#1867
