WIP: Flexible and consistent icon sizing

The current issue is we have no standard for icon sizes. Different areas and docker use different hard-coded icon sizes. All these different sizes make the UI a bit sloppy to look at in places

Bring more consistency by having the icon sizes use a configuration from KisIconUtils. Some areas like the title bars might not update because of how that widget is coded. That will have to be a separate patch.

Here is a guide that Tim came up with for some rules on what areas should have different icon sizes https://phabricator.kde.org/T14100

Test Plan

Start Krita. Change the icon size from the toolbox context menu. Right now the only icon that I am updating is the view options button in the brush presets docker. The size should update.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.