Add the ability to delete text in the markdown.

[dalyIsaac]

• [ ] Deletion on <p>C</p> resulting in an empty string should convert it to <br /> (Note, C is any random content). #63
• [x] Deleting single characters using the Backspace key #59
  • [x] Fix for empty new lines
  • [x] Fix for populated new lines
• [ ] Deleting single characters using the Delete key #60
• [ ] Selecting multiple text, and deleting #61
• [ ] Selecting multiple text, and inputting new text #62

[dalyIsaac]

When deleting a character using Backspace, when the selection is the start of a StructureNode:

• if there is no StructureNodes before the node which has selection, then nothing should happen.
• if there is a StructureNode before the node which has selection, then the two StructureNodes should be merged.