screenchooserdialog: make main text reflect what the dialog is showing

Right now it always asks you about screens if the app name is not available, even if it's actually asking about windows or screens or both. It also does not take into account the number of things it's asking for or whether multi-selection is allowed.

Now it does.

cc @teams/usability

	Before	After
Asking for screens
Asking for windows
Asking for both