Improve settings code reability

Application::SetSettings relies on Application::GetSettings to
initialize the application's settings if this hasn't happened.
This means that GetSettings is being used not to get the app's
settings, but to initialize the settings!

I have corrected this by introducing a private static function
InitSettings(). Note that I assert if it attempts to initialize
already initialized settings, because the (Get|Set)Settings
does the check for you.

Also changed a local variable from nGet to nDragMode in
Desktop::SystemSettingsChanging to make it easier to read the
code. Whilst this is minor, a variable name of "nGet" seems
very sloppy.

Change-Id: I04975217c2028b3489179997db4287957578cc93
Reviewed-on: https://gerrit.libreoffice.org/7586
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>