Fix a broken signal/slot connection in KPrefsWidDuration().

This was effectively blocking the KOrganizer HTML autoexport from
ever being run.

Also don't continually blast the user with prompt and info dialogs
when in autoexport mode.

...and a few other nice things for the UI experience.

BUG: 302668
MERGE: 4.8,4.9
FIXED-IN: 4.8.5
(cherry picked from commit 02911410)
(cherry picked from commit 9ac33235)

Conflicts:
	korganizer/htmlexportjob.h