Better tab support
This adds better tab support for the editor:
- Possibility to choose between "classic" tab and spaces instead of tab (default to 4 spaces)
- Tab/backtab of selected text will apply the effect to the whole block of line
- When pressing return to get a new line, the new line will now follow the indentation of the previous one. It will also add a new list item if the previous line was one.
This also moves most of the "TextToolBar" logic code to C++ inside 'MDHandler"