[addon closebrackets] Add smart backspace behavior

When any specified pair is empty, backspacing will also
delete the closing bracket.
[closebrackets addon] Simplify backspace-handling code
Add hasFocus method
[placeholder addon] Add
Remove unused variable
Set a default background and color

Closes #1346
[closetag addon] Fix bug in self-closing tags typed directly in front of word

Closes #1349
Remove a debug statement
Don't try to scroll if editor isn't visible