Make sure that all code uses MyMoneySplit::Action... variables instead of duplicated strings

Summary:
This patch is a preparation for adding MyMoneySplit::ActionSellShares and
helps to avoid regressions by not patching all related locations.

CCBUG:394124

Test Plan: compiled on linux

Reviewers: tbaumgart

Reviewed By: tbaumgart

Differential Revision: https://phabricator.kde.org/D12917