Provide a button to quit Inkscape from the Welcome Screen

Summary:

When opening Inkscape and arriving at the welcome screen, there is no way to quit Inkscape without force-quitting it through your operating system's UI. This is kind of annoying, especially when Inkscape is opened accidentally.

Steps to reproduce:

• open Inkscape

What happened?

The welcome screen opens with the options to Load or New Document only.

What should have happened?

IMO there should be another option to Quit.

Sample attachments: N/A

Version info

Inkscape nightly from PPA.

Inkscape 1.2-alpha1 (1:1.2.0-alpha1+devel+202204062350+82c74e9b90) 

    GLib version:     2.68.4
    GTK version:      3.24.30
    glibmm version:   2.64.2
    gtkmm version:    3.24.5
    libxml2 version:  2.9.12
    libxslt version:  1.1.34
    Cairo version:    1.16.0
    Pango version:    1.48.10
    HarfBuzz version: 2.7.4

    OS version:       Ubuntu 21.10