Adding Docker 1.12 instructions to its grammar

Settings: view state lost when switching tabs 

fixes #16258

* fix #15698

* better fix to keep watermark visible

* react on drag enter, not drag over

Typo: "stage" is incorrect action because it wont do anything, "stash" is. 

Fix #15705.

This simply updates the Docker grammar to include the `HEALTHCHECK` and `SHELL` commands that were added to Docker 1.12, and were added to the [grammar file](https://github.com/docker/docker/blob/4cb71f80823af345d063cf0ad657e73ce9caa75f/contrib/syntax/textmate/Docker.tmbundle/Syntaxes/Dockerfile.tmLanguage) which VS Code's copy is based on.

Handle backups in the event of deleted workspaces

Fixes #16446

We want backups to always happen in case of a crash.

Fixes #12327

Notify the user that backups exist but have not been restored if the folder to be restored no longer exists (fixes #14705)