Draft: Improve activities support

Besides fixing KActivities detection, this adds support for closing windows when they are only shown in stopped activities and restore them when one of those activities are started again. This is obtained by storing information about windows in stopped activities in a separate configuration file, which contains a list of windows in each stopped activity and the information to restore such windows (as produced by KonqMainWindow::saveProperties. To distinguish each window in a permanent way, an uuid is added to each window.