Backport the sort order fixes. I reverted all string changes, those are not very visible strings anyway. Sorry for the very very late commit, I hope I didn't break anything and apologize in advance. ===

Backport r909096 by tmcguire from trunk to the 4.2 branch:

Remove the sort order settings from the aggregation, and make
a stand-alone class instead, that can be saved per-folder or
globally (the default).
This should fix the problems with the message sort order not
being remembered.

Also, fix a little bug that the sort indicator of the headers
would not always be shown.

CCBUG: 175643

Backport r913236 by tmcguire from trunk to the 4.2 branch:

Since I decoupled the sort order from the aggregations, there
were some combinations that didn't made sense, for example grouping by
date by sorting by something different.

Fix this situation by not allowing those combinations, and using
sensible defaults instead.

Backport r913240 by tmcguire from trunk to the 4.2 branch:

Slightly better way to detect incorrect sort orders, as we now know the default value.

Backport r913666 by tmcguire from trunk to the 4.2 branch:

Fix crash.

svn path=/branches/KDE/4.2/kdepim/; revision=913714