Make sure to save security settings each time they are modified

Settings that are modified using normal settings window, which is
invoked using menu "Configure Desktop Sharing..." are always saved
correctly.

However some settings that are present on Krfb main window
(normal password, unattended password, checkbox "enable unattended
access") are not handled by KConfigDialog and there is a chance that
settings might not be saved correctly (for example if application
was suddenly terminated).

This hopefully fixes some bugs which are present since year 2011-2014
and have 110 votes, like https://bugs.kde.org/show_bug.cgi?id=340411,
maybe something else.

BUG: 340411
CCBUG: 288481
CHANGELOG: Make sure to save passwords each time they are modified