Md checkbox action

[jdckr]

Hi,
I have added a button for checkbox action:

• if there's no checkbox - [ ] is added
• if line stats with - [ ] it is replaced with - [x]

Furthermore I adapted MarkDownAutoFormat.java as auto insert of list didn't work for - [ ].

[gsantner]

@jdckr You recently worked on that area, I think honestly you know better how it works now :=).
I tried it and have not noticed something major. Only found one issue, that is if you have a list, and go nested it always starts with beginning of line (see example below). This can be resolved later too and I'm fine with this happening for now. Merging ;). Thanks for your repeated contribution, always happy about help and hope you continue 👍 .

Example (with underscore being a space):

Currently on git-master:
- text
- more text
  - subitem press enter
  - this is the new item


Now:
- text
- more text
  - subitem press enter
- this is the new item