Change dialog while extension output is being loaded

Summary:

On Windows, Inkscape can be unresponsive for a long time after an extension has finished (see issue 3144). However, the dialog that is displayed is "'ExtensionName' working, please wait". This is misleading, as it causes the user to believe that the extension is the issue, not Inkscape.

Until issue 3144 is resolved, it would be more accurate to display something like "'ExtensionName' has finished, loading file".

Version info

Inkscape 1.2.1 (9c6d41e410, 2022-07-14)

    GLib version:     2.72.2
    GTK version:      3.24.34
    glibmm version:   2.66.4
    gtkmm version:    3.24.6
    libxml2 version:  2.9.14
    libxslt version:  1.1.35
    Cairo version:    1.17.6
    Pango version:    1.50.7
    HarfBuzz version: 4.4.1

    OS version:       Windows 10 21H1