[Lehonti]

Follow-up on #2327.

Almost all configurable effects in Pinta override IsConfigurable => true and launch SimpleEffectDialog as their configuration UI.

This change adds a ConfigurableEffect base class that sets IsConfigurable => true and launches the simple effect dialog by default in LaunchConfiguration (ILivePreviewSession).

Also, BrightnessContrastEffect is updated to inherit from it and serve as a sample of how it would work. Once the remaining configurable effects are migrated, the parameterless LaunchConfiguration overload on BaseEffect can be removed.

And I am considering that later we could pattern-match instead of checking the IsConfigurable property, for better type safety.

[cameronwhite]

Looks good to me 👍 My only suggestion would be adding some brief doc comments to the class since this will be used for add-ins

[Lehonti]

@cameronwhite I added a short description explaining the default mechanism