On macOS 10.12 Sierra, Apple changed the behaviour of CFPreferencesSetValue()
truncating data after a null character. https://bugreports.qt.io/browse/QTBUG-56344
Due to this, we have to move from native plist to IniFormat.
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.
