Add option to enable fixing of the buy/sell signs

Some providers do not follow the OFX spec and do not assign the correct
positive/negative signage to amount (TOTALs) and quantity (UNITs)
values assosciated with BUY/SELL investment transactions (see OFX2.2,
section 13.3.3). This change adds option to OFX import dialog and
account's online settings to force the signage to follow the the spec.

BUG: 420056
GUI: