Continously increasing memory consumption when running extensions

Migrated from https://bugs.launchpad.net/inkscape/+bug/1194871. Contains a patch.

Summary:

When running extensions, there is a possible memory leak.

Steps to reproduce:

• open Inkscape. Task manager shows a memory consumption of 130 MB.
• Run the Render -> Grid -> Isometric grid extension. It doesn't matter which one, but this one creates a lot of objects, so it's good for our test.
• Undo (Ctrl-Z)
• Run the extension again (Alt+Q)
• Repeat the last two steps a few times.

What happened?

Every time the extension is rerun, the memory consumption is increased by about 5 MB.

What should have happened?

The memory consumption should stay more or less stable.

Version info

Inkscape 1.2-alpha (0bd5040e63, 2022-02-05)

    GLib version:     2.70.3
    GTK version:      3.24.31
    glibmm version:   2.66.2
    gtkmm version:    3.24.5
    libxml2 version:  2.9.12
    libxslt version:  1.1.34
    Cairo version:    1.17.4
    Pango version:    1.50.3
    HarfBuzz version: 3.2.0

    OS version:       Windows 10 20H2