1. Extract "transaction" support for QSettings into separate class
TransactionalSettings.
2. Define macrto with explicit name for the case when this "transaction"
support is needed.
3. A bit optimize QHash <-> QSettings copying: replace assign with
insert() and remove repetitive key lookups.
4. In save() check dirty status before getting the lock too.
The changes from items 1 and 2 make text more structured and the logic
of the SettingsStorage class gets separated from the implementation level
task of guarding the settings serialization. The changes in 3 and 4 do not
make the app much faster, but neither make any harm to the code readability.
