[Gimp-gui] Save Button to be Added to Exit Menu

[56independent <56independent protonmail com>]

Whenever I exit the program, I often get a little popup that says
​"you have unsaved changes!". This is rather inconvenient, as it has
no save button. Just "Help" "Cancel" and "Discard Changes". I propose
adding "Save and Exit" to the list. Like Inkscape, if you no longer
want to edit a file, and want to do other things, you can then just
quit, with minimal obstruction. The current system forces me to do
"esc" then "crtl+s" then "alt+f4". Inkscape allows me to do "alt+f4"
"tab" "tab"   "tab" "enter" to leave.