All programming text editors have "Tab" key to insert indentation.
Additionally, while discussing code related changes, we might need to
insert whitespace, and remarkup also uses tab for code.
This patch makes the tab key simply indent 4 spaces. However it
keeps this optional and can be enabled by a new panel in the preferences.
If this is accepted I will add additional features like:
- Preserving indentation when "Enter" is pressed
- "Shift+Tab" de-indenting the line