4 bugfixed

• bugfixed: Opening the volume again after reading over 10 pages resulted in an unauthorized termination
• bugfixed: Keyboard input did not work if FolderWindow is displayed in the main window
• bugfixed: The image was not loaded even if switching the volume with the keyboard in FolderWindow
• bugfixed: After switching to another Volume with SpreadViewing, and go back to original Volume, it goes one page
• changed: for including some Qt common headers