#738 GUI: Size and margin seetings ignore integers

[Tiger Beard]

System: Ubuntu22.04, repository package gimp-gutenprint (5.3.3)
conirmed on Ubuntu17.04, repository package gimp-gutenprint (5.2.13)
Printer: Epson Stylus Pro 4880 (but should be unrelated to the printer model)
Values set to "mm"

Observed:
Set paper size to "custom" and enter the dimensions to 420 (no return) and 600 (no return). Change orientation to another value (e.g. to Landscape). The size values are replaced by the previous values.
Do the same and press "return" each time. Now the values are accepted.
Set all values again and press "Save Settings". Close dialog and reopen. The settings are gone.

Set Left to 10 (no return), press TAB and set Top to 5 (no return). Set paper size to "A4". Both values are lost and reset (in my case to 0.0.)
Set all values again and press "Save Settings". Close dialog and reopen. The settings are gone.

Expected:

• when a textbox is set to a new value and the textbox is left the value is accepted. Its unexpected behaviour to the user that a textbox needs RETURN to apply the value. Other gimp textboxed are applied when another GUI element is selected. Example: Rotate about angle, set the value to 10 press TAB. The angle is accepted as "10" and the screen updated.
• when "Save Settings" is selected all current settings in the text boxes should be saved.

Reason:
unexpected behaviour for the user