Keybindings Support

[voldyman]

this branch should allow the user to easily insert markdown equivalent tags on keypresses, so pressing should insert the italics tag [cursor here]

• Keybinding settings class
• basic layout for the schema
• make the Document widget listen for keypresses and check if it should handle it
• load keybindings from settings
• reload keybindings if they are changed
• if text is selected insert the tag around it, selectedText → ctrl i → selectedText