Smart Editor: add [+] button

[Drugoy]

I suggest to add [+] button that should appear at left from code's line numbers for the hovered line. Clicking that button should add 1 new line into the %rule% block (e.g. after "{", but before "}") just after the hovered line. If the hovered line contains "}" - the new line should appear BEFORE it but after the last %subrule% inside the %rule% block.