Fix session management with preloaded windows

Session management stopped working corectly with preloaded windows when
Konqueror was ported from having many application instances to a single
application with many windows. The session management code wasn't
changed accordingly, which caused preloaded windows to be restored as
full windows.

To correctly exclude preloaded windows from session management, we use
KMainWindow::saveGlobalProperties to store the indexes of preloaded
windows and KMainWindow::readGlobalProperties to read them back, so that
we know which windows should not be restored.

This commit also refactors much of the startup code, moving it from
main() to KonqApplication.

BUG: 450624


(cherry picked from commit 26d98061)