Merge querypage-work2 branch from trunk. The most relevant changes are:

Merge querypage-work2 branch from trunk. The most relevant changes are:
* QueryPage now uses array-based query building instead of raw SQL
* Converted all QueryPage-based special pages that were using old-style wfSpecialFoo functions to new-style SpecialPage subclasses; this is possible because QueryPage is changed to extend SpecialPage
* Backward compatibility for extensions is partly preserved: getSQL() is fallen back on for QueryPage subclasses that don't implement getQueryInfo(), but getOrder() will be ignored (implement getOrderFields() instead). This also means that dual compatibility (1.18 compat and b/c with pre-1.18) is trivial

Extension changes will be merged after this commit.

These changes make it easier to write an API module for QueryPages (bug 14869); this wasn't done in the branch but will be done in trunk soon.