Indicator for Vim plugin loaded/mode desirable

## Problem
The Vim plugin gives no indication (a) that it is loaded or (b) what mode it is in

## Proposal
When the Vim plugin is loaded, display an icon in the headerbar in the same way the Terminal plugin does. The icon should change depending on whether the plugin is in COMMAND, INSERT or REPLACE mode

## Prior Art
The Vim app shows the words "INSERT" or "REPLACE" on the last line when not in command mode.
Most other GUI apps have some kind of status line showing the mode.