Adjust all icon sizes for GIMP

Hi,

I’ve worked on a MR to adjust all icon sizes for GIMP users in 4K, or for those who would like, or need, larger icons.

It’s a sprawling MR, that tweaks a lot of individual icons, a kind of retro fitting. The MR could be a lot simpler, if the functions throughout GIMP could access a couple of values as if they are global variables. This may not be doable, however a technique might be obvious to a software engineer.

The user sets two values through the ‘Preferences’ GUI in GIMP, which are stored in the ‘gui_config’:
gui_config->custom_icon_size
gui_config->override_icon_size

of types:
gboolean
GimpIconSize

they are also saved in gimprc:
override-theme-icon-size yes
custom-icon-size large

Currently the MR has to pass around, or find a path to the ‘gui_config’, to then adjust the icon sizes. Which seems inefficient, and much yuck. If thou can help, please illuminate, thank you.