Add feature to disable the splash screen

BUG:367787
FIXED-IN:5.0

Changes:

1. Single unique pointer to splashscreen instead of two pointers.
2. Class has been changed to simple function, so it takes less space.
3. logoOverlay.isNull() is taking processing resources, while splashscreen can be created with empty pixmap, so that check was removed.
4. Splashscreen can be disabled.

Differential Revision: https://phabricator.kde.org/D6422

Signed-off-by: Łukasz Wojniłowicz <lukasz.wojnilowicz@gmail.com>