Correct display problems in mail request dialog

1) improve coordinates display and ensure they can be typed even with big font size
2) clarify and complete error messages in case mail area is over limits ( 2mb for zygrib, an area with sides under 180° to ovoid enormous files and an area less than two time the resolution to ovoid ridiculously small area)
3) the most important : with big fonts size and/or small screens the request dialog was only partially visible and the buttons inaccessible . Fixed by adding a scroll bar when the dialog size is over the screen size