Pop up a dialog box and return user's selection.
POSITION specifies which frame to use.
This is normally a mouse button event or a window or frame.
If POSITION is t, it means to use the frame the mouse is on.
The dialog box appears in the middle of the specified frame.
CONTENTS specifies the alternatives to display in the dialog box.
It is a list of the form (DIALOG ITEM1 ITEM2...).
Each ITEM is a cons cell (STRING . VALUE).
The return value is VALUE from the chosen item.
An ITEM may also be just a string--that makes a nonselectable item.
An ITEM may also be nil--that means to put all preceding items
on the left of the dialog box and all following items on the right.
(By default, approximately half appear on each side.)
If HEADER is non-nil, the frame title for the box is "Information",
otherwise it is "Question".
If the user gets rid of the dialog box without making a valid choice,
for instance using the window manager, then this produces a quit and
`x-popup-dialog' does not return.
(fn POSITION CONTENTS &optional HEADER)