Make MyMoneyFile header less dependent

The goal of this patch is to reduce dependability on mymoneyfile and of
mymoneyfile on other libraries.
Changes:

1. no header file includes mymoneyfile now,
2. mymoneyfile.h uses forward-declarations instead of includes.

Patch is huge but it's nothing but code shuffling. Nonetheless it has
two important merits i.e. smaller executable file and shorter
compilation times.

Differential Revision: https://phabricator.kde.org/D8535