## Summary of the Pull Request
Adds a pair of `ShortcutAction`s for setting the tab color.
* `setTabColor`: This changes the color of the current tab to the provided color, or can be used to clear the color.
* `openTabColorPicker`: This keybinding immediately activates the tab color picker for the currently focused tab.
## References
## PR Checklist
* [x] scratches my own itch
* [x] I work here
* [x] Tests added/passed
* [x] https://github.com/MicrosoftDocs/terminal/pull/69
## Detailed Description of the Pull Request / Additional comments
## Validation Steps Performed
* hey look there are tests
* Tested with the following:
```json
// { "command": "setTabColor", "keys": [ "alt+c" ] },
{ "keys": "ctrl+alt+c", "command": { "action": "setTabColor", "color": "#123456" } },
{ "keys": "alt+shift+c", "command": { "action": "setTabColor", "color": null} },
{ "keys": "alt+c", "command": "openTabColorPicker" },
```
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