Typed shortcuts

Summary: The user can type directly in the textarea the widgets' shortcuts and they will be replaced on the fly by the widget placeholder. Also, widget insertion in mid-text!

What's been done:
* Added three new Util's methods for handling text manipulation and parsing
* Removed the onKeyUp handler
* Changed the onKeyDown handler to reflect the new solution
* Changed the addWidget method so that it can insert the widgets where the cursor is instead of just at the end of the content