renaming

- CHG: SurroundTextEventHandler => TMSurroundTextEventHandler
- CHG: SurroundTextEventArgs => TMSurroundTextEventArgs
improvments, fixes and documentation

- ADD: missing documentations
- CHG: improvments
- FIX: tag output
buttons and cleanings

- ADD: buttons for bold, italic, underline and strike through styles
- CHG: cleanings
HTML parsing

- CHG: space chars are replaced with &nbsp; entities now
loading and saving documents

- ADD: article state can be saved and loaded from document file now
cleanings

- CHG: converted base view model of PreviewWindow.ViewModel class from
field to property