Add hidden configuration option "Open Images in the Same Window" which, when enabled, forces "File / New", "File / Open ..." and "File / Open Recent" to never open a new window.

Given that it violates the KDE UI standards, I don't think it should have a user-visible config option - do you agree or disagree?

Testing has been limited due to my checkout of kdelibs 4.0 having far too many bugs.

Technical details:
- Removed kpMainWindow::shouldOpenInNewWindow() and inlined the code as the method would be too confusing if this configuration option was on

Support for the feature:

1. BUG: 125116

2. Frank Wieduwilt, Linux Magazine Issue #44 (Jul '04) p77:

"Unfortunately, the program [KolourPaint] opens up each image in a separate window. This tends to clutter up the screen while working with multiple images. The developers have promised to change this in the imminent 1.1 version, however."

3. And the feature was also silented requested by me :)