select the initial hint on backspace key press
clear hints when nothing's left
fix the bug that prevents Ctrl+Space from invoking css hints
Merge branch 'master' into css-code-hints-improvements

Conflicts:
	src/extensions/default/CSSCodeHints/main.js

"Select the initial hint on backspace key press" has been done by @redmunds on master, fixed conflicts
reapply 'select the initial hint on backspace key press'
Merge pull request #6242 from YuAo/css-code-hints-improvements

CSS code hints improvements