Reduce size of spin button icons

After researching various methods and what other programs are doing, we place some fresh icons in our themes to control how the add and remove icons look to our users.

Using the system/computer theme would lead to unpredictable results, especially on macOS and windows. But also on many linux systems (for example Papyrus icon theme)

Gimp ships their own icons for these, so with that example in mind:

Fixes #2891 (closed)

BEFORE:

AFTER: