Add status bar icon for QML Language Server

Sometimes users cannot understand whether QML Language Server is activated or not. We can add a status bar icon that shows the status of QML Language Server.

Cases:
1. QML Language server is disabled explicitly
In this situation, the icon shows a X indication that it doesn't work
2. QML Language server works
In this situation, the icon shows a Tick
2. QML Language server crashed or cannot run
In this situation, the icon shows a ! and letters turn yellow.

When the icon is clicked, it will show a quick pick item with Enable and Disable
or it will show the qt-qml.qmlls.enabled setting in UI.

https://code.visualstudio.com/api/ux-guidelines/status-bar