Always clear selection when leaving visual mode.

Improve the original test to see if the selection is cleared when
leaving visual mode; there was no actual leave command triggered.

Test that entering command line mode from visual mode (thus using a line
range) will go back to normal mode AND clear the selection.

Tweak the commond parser to run an extra LeaveVisualModeCommand when
coming from visual mode to fix the latter test. Running it
unconditionally would have side-effects: quitting command line mode
without a selection would clobber the last selection marks and "gv"
command.

Fixes #399.