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.

首先比较 Terminal plugin 如何显示其 headerbar indicator，以及 Vim plugin 的加载和模式处理方式。加载后的 Vim plugin 显示 headerbar icon，并且该 icon 会针对 COMMAND、INSERT 和 REPLACE 模式发生变化时，工作即告完成。

由索引模型根据 Issue 内容生成。