[CustomItemColors] Added disable option, refactored code, added support for more font colors

[Caedendi]

Previously there was no option to disable changing a font color. Selecting the default value would overwrite the current value, which is both unnecessary and would conflict with other mods.

This pull request changes:

• new Disable option for each setting: selecting this will skip the font color.
• clarified the Game Default values with a (force) message: selecting this will force-overwrite the current value with the game default value like the previous version did.
• added support for more color variables: HealthPotionColor, ManaPotionColor, RejuvPotionColor, RuneColor and EventItemsColor.
• Refactored the code

[Caedendi]

If you like the feature changes but don't like the code style after the refactor, I could redo it in the old style if you'd really prefer that.

[olegbl]

Default (equivalent of this PR's Disable) option added by 78d10aad8068bd8df8a67e8a03381219fb5c32f6. Additional colors added by bef8e1692d6b9a3c8480ecdd4dc6b545836d325b.