Add theme support using Qt (css) style sheets.
On branch master, changes to be committed:
	modified:   config/theme.conf
	new file:   config/themes/defaultTheme.css
	modified:   src/generalconfig.cpp
	modified:   src/generalconfig.h
	modified:   src/mainwindow.cpp
	modified:   src/scintillaeditor.cpp
	modified:   styles/default.xml
Optimize and improve default theme. Fix bug of closing editor window.
On branch master, changes to be committed:
	modified:   config/theme.conf
	modified:   config/themes/defaultTheme.css
	modified:   src/mainwindow.cpp
	modified:   src/scintillaeditor.cpp