[Draft] Create numbered and bulleted lists

This PR aims to add the creation of lists in a document. This only aims to support numbered and bulleted lists and not image bullet lists for now.

# TODO
* [x] Editor UI buttons
* [x] Detect list style at selection
* [x] Ops for list insertion/removal
* [ ] OT support
* [ ] Convert selected text to a list or selected list back to text
* [ ] Default styles for numbered and bulleted lists

Class should expose it's own set of signals for subscribers to use. Similar to the direct formatting controller. E.g.,: 
```
/**@const*/gui.DirectFormattingController.enabledChanged = "enabled/changed";
/**@const*/gui.DirectFormattingController.textStylingChanged = "textStyling/changed";
/**@const*/gui.DirectFormattingController.paragraphStylingChanged = "paragraphStyling/changed";
```