Main window should center on screen

When update manager opens (automatically or otherwise), the window is placed in an often unpredictable location, such as in the upper left or bottom right corners of the screen. This can make it slightly difficult to find the window when switching focus to Update Manager when it was opened automatically in the background, and this apparently arbitrary positioning can confuse the user in general. If the main window were always centered on screen, it would be easier to find the window when it is switched to, and it also makes the window seem more important and commands the user's attention.