Change ToggleLineComment behavior.

A problem would occur where a preexisting LineComment would become uncommented if ToggleLineComment was repeated twice.
To resolve this, I altered the behavior so that a preexisting LineComment will be doubly commented out, like many other editors.
Merge pull request #140 from ton1517/linecomment

Change ToggleLineComment behavior
Update build.cmd
Merge https://github.com/fdorg/flashdevelop into QuickNavigatePlugin