Add totals row on table reports

Patch changes the way subtotals are delivered to the screen. Instead of
calculating subtotals in renderer, renderer identifies specific row as
totals row.
Things changed:

1. multiple subtotals possible instead of only one,
2. radesigned rank variable as follows 0 - opening balance 1 - major split of transaction 2 - minor splits of transaction 3 - closing balance 4 - totals row
3. GroupIterator has been replaced by simpler code without any hacks
4. in case of multiple currencies only grand total isn't displayed and

not all totals

5. Investment Performance and Capital Gains report have all columns

subtotaled

BUG:314021
FIXED-IN:5.0

Differential Revision: https://phabricator.kde.org/D5099

Signed-off-by: Łukasz Wojniłowicz <lukasz.wojnilowicz@gmail.com>