Allow sorts to have implicit filters

This one isn't of much use via the UI, but it's very useful for developers building data programmatically with Views. In particular, the Custom Pagers module: I need to run the View twice, once getting entries *higher* than a particular one in a sort, then again getting all values *less* than another.

Not all 'sort' handlers have an accompanying 'filter' handler, so programmatically adding a filter for each sort isn't possible. This code (suggested by merlinofchaos) adds a $sort->filter($operator, $value) function to the default sort handler. It would apply the sort as usual but also act as a built-in 'cutoff' for values above or below a particular spot in the sort. This is not currently exposed in the UI, but exists so developers doing similar things can quickly slice-and-dice a sorted view even if there are no matching filters defined.

Code needs testing, I was about to test another set of patches and wanted to make sure I rolled this and posted it so it didn't get munched when I applied subsequent patches.