This change implements a QucsSingleton class that can be used to
make any class singleton. It also provides a new settingsManager
class which uses QucsSingleton to provide a single, static access
point to the standard QSettings functionality. A shortcut is provided
and a new method for providing a default value if the requested key
is not set.

…ties

The recent docs list is stored as a * separated string and needs to be
converted to a string list when reading from the settings file.

Replace almost all calls in main::saveApplSettings to the new unified
settings model.