Add accessibility settings, implement image preview field

[salianifo]

Adds a new accessibility settings page with the following settings:

• Image preview height [default: 50 px] (used for the image preview field)
• Event block bottom margin [default: 0 px] (used to add some extra space between event blocks to improve readability)
• Show event names in timeline [default: false] (adds a header label to the beginning of all events with the event name to help distinguish events better)

The image previews are automatically unloaded when the event block body is hidden, and not loaded at all if image preview height is set to 0. Image preview size and event block bottom margin are scaled according to the editor scale (DialogicUtil.get_editor_scale()). The settings page includes hints noting that some settings require reloading the visual timeline for changes to take effect.