Set all dialogs to open in the center of their parent

[bparks13]

In previous commits, panels were updated to eliminate the percent scaling issue that was being seen; while this is fixed if the dialog is opened in the same screen as the original Bonsai window, it is not correct if Bonsai is in a window with a different screen resolution than the window that the dialog is opened in.

The fix is easy; set StartPosition to CenterParent instead of the default for all dialogs that can be opened from Bonsai. This will ensure that the same resolution is used for the new window, and it should not have any issues with scale.