help: updated screenshot for the Main Window

This one needs no translating as it uses numbers instead of labels.
help: Mention "menu button" rather than "app menu"

"App menu" could be confused with the global menu that you find in GNOME Shell
help: Be realistic about expectations of supporting legacy capture technology
help: Remove the GTK+ FileChooser screenshot

It's not extremely important, and the FileChooser changes often these days
help: Remove the "hovering tooltips" screenshot

The screenshot is very old, and tooltips are kind of self-explanatory.