Add --position argument to position the dialog

Add a position option --position which allows you to specify the position of the dialog, e.g.,

• +100+50 - 100 pixels from the left and 50 pixels from the top.
• -120-50 - 120 pixels from the left and 50 pixels from the botom.
• =50%=50% - in the center.

Applies this to all dialogs, except for notification - I wasn't certain how this one worked.