feat(okular): More accessibility options

[lordkekz]

Changes

• Fixed a typo in performance.memoryUsage enum (Aggressive should have two "g").
• Expanded RenderModes for ChangeColors, as they were incomplete.
• Added threshold and contrast options for the Black&White mode as applicable.

Testing

• I investigated the enum values using Okular 24.02.2.
• I tested the new Black&White detail options also using Okular 24.02.2.
• Comments on performance.memoryUsage enum are based on KDE Documentation.

[HeitorAugustoLN]

Thanks for your contribution! Fixed some minor things but everything else looks good :)
I'll merge it as soon as CI passes.