Introduce tab width settings to options

Conflicts:
	app/optionsdialog.cpp
	app/optionsdialog.ui
Moving handler for tabWidthChanged to editor class. Fixed coding style error. Used QFontMetrics to determine tab width in pixels.
Merge branch 'tabwidth-changes' of https://github.com/Slesa/CuteMarkEd into slesa-tabwidth-changes