The related classes doesn't really need recursive mutex so drop it. And
if it is really required it usually indicates bad design and we should
avoid it anyway.
We should not blindly remove the leftover settings file, as the following write()
operation could fail and the user would lost all settings.
We should try renaming it instead.
It may be useful to have different configurations either for portable
versions or for debugging purposes. To implement this we add two
options, avaliable via command line switches
1. An option to change configuration name ("--configuration"). The name
supplied via this option is appended to
QCoreApplication::applicationName() to form "qBittorrent_<conf_name>"
name for the configuration files.
2. An option to provide a path do directory where all the settings are
stored (kind of profile directory). There is a shortcut "--portable"
which means "use directory 'profile' near the executable location".
In order to implement that we have to perform initialisation of the
profile directories before the SettingStorage and Preferences singletones
are initialised. Thus, options parsing shall be performed without defaults
read from preferences.
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.
