Allow __group__ reports to be sorted

[Christian Boos]

This is a direct follow-up to #10530, targeted to 0.13dev.

The change aims at fixing the problem solved in r10923 in a more satisfying way:

• instead of disabling the sort by user selected column feature in case the report uses a __group__, fix this feature by respecting the __group__ ordering first, then sort by this user selected column,
• the approach to paging done in #216 relied on a subquery, but in fact there's no guarantee that the order of the rows in the subquery is kept at the level of the main query… we should fix this by not using a subquery.

See the discussion there for full details.

There might be more discussion needed and follow-up changes, hence this ticket.

[Christian Boos]

make it possible to specify where to insert the sort column

[Christian Boos]

Branch rebased and applied in r11096.

In fact, I still need to update the 0.13/TracReports page as promised in the error hint in that code…