Squashed commit of the following:

commit 70b0382e
Author: Johannes Rieken <johannes.rieken@gmail.com>
Date:   Fri Jan 24 16:41:06 2020 +0100

    update statusbar with "real" keybindings

commit d413a30a
Author: Pine Wu <octref@gmail.com>
Date:   Fri Jan 24 15:31:55 2020 +0100

    Polish

commit a1bd86cc
Author: Pine Wu <octref@gmail.com>
Date:   Fri Jan 24 12:25:32 2020 +0100

    Simple StatusBar for suggest

Fixes #88229

There should a 1:1 mapping between VS Code's custom editors and custom editors (specifically the webviews) that extensions know about. This was previously not true as a single `CustomEditorInput` could end up being shared between multiple editor groups.

With this change, we always create a copy of the custom editor input during split, and then add a listener for `onDidCloseEditor` so that we can make sure we get rid of the unique inputs (and notify extensions of this)

Allows extension install and uninstall command execution to throw exceptions back to caller