Align and distribute: Increase icon size

This is a regression bugfix. In older versions (<= 1.0), the align and distribute icons were rendered as 24px wide. With newer Inkscape and GTK, this changed to 16px, which makes the icons difficult to distinguish, especially with the new dark symbolic icon theme.

The old size of 24px occured as a "technical accident": The button had the default size of 16px, but because the alignment icons are not available in 16px a larger icon was used. Gtk changed this behavior at some point and started scaling down the icons. With the new symbolic theme, 16px icons also became available.

Now, a size of 24px is specifically requested. This restores the previous icon size as it has been with Inkscape 0.92 and 1.0.

Related: inbox#6491 (closed) (panel icons are too small for some users)

Related: inbox#7283 (16px is way too small for screens with slightly higher DPI (e.g. 150% of the standard 96DPI)

Happy holidays to all people who have some 🎄