Allow titles to be centered relative to the full window title.

Previously, the title would be centered relative to the space between the window controls and icons. This would cause the centering to look odd when other center-aligned content was displayed in the window.

This commit adds a check box to the Window Manager preferences window that allows the title bar to be centered relative to the full width of the window.

Since this is an optional feature, it shouldn't break compatibility with certain themes and user preferences.