1. Use unified function names `initInstance()` and `freeInstance()` and
make them public.
2. Add `freeInstance()` to avoid noise from memory leak detectors.
3. Let `instance()`return a pointer directly to avoid unnecessary
indirections when invoking functions.
Conditionally change absolute paths to relative in the fastresume data files.
The condition is specified by user via a command line parameter and
paths are relative to the profile dir.
On Windows the convertion to relative path is performed if the path and
the profile are on the same drive only.
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.
