Fixes to highlight and small refactoring

[blueyed]

I've tried to improve issue 1150, and found some minor bugs, e.g. :hi clear after :hi Foo color:red did not clear/remove Foo.

blueyed@6c1954f was meant to improve performance, but without (much?) success.

Is there a way to profile :hi clear?

[prikhi]

@blueyed Thank you for mentioning :hi clear! I haven't used multiple pentadactyl windows for months because of the lag and random shutdown when opening new windows!

I tried profiling using the gecko profiler: https://developer.mozilla.org/en-US/docs/Mozilla/Performance/Profiling_with_the_Built-in_Profiler

But some time after I run :hi clear and the window freezes up, firefox just quits on me so I can never see the results :(

[blueyed]

@prikhi
btw: I am back to using Vimperator currently. There is a related issue at vimperator/vimperator-labs#45.