Remove virtual keyword from some functions.

These functions are not overloaded at the moment in any child class but
some of them are called very often which adds a significant overhead. If
really needed they should be reverted to virtual when we actually
overload them.
Add lazyness to filterAcceptsRow
Only run the loop until the condition changes
Less calls to itemFromIndex
First do visibilty check to reduce branching
Memoize visibility filter
Various small performance improvements
Merge pull request #265 from tomahawk-player/playableproxyfilter-performance

Playableproxyfilter performance