Ability to control visibility of features and tags via sidebar

[reckart]

Is your feature request related to a problem? Please describe. It is currently not possible to selectively show or hide individual features or tags within the layer sidebar. This limits the user's ability to focus on relevant aspects of the annotation data, especially when dealing with complex layers or tagsets.

Describe the solution you'd like Extend the layer visibility sidebar to display the features of each layer beneath it, along with controls to toggle their visibility individually.

• [x] Display features of each layer below the layer in the sidebar
• [x] Allow toggling visibility of individual features
• [x] Hide features that are toggled off in annotations
• [x] Display tagsets under string features
• [x] Allow toggling visibility of individual tags
• [x] Hide tags that are toggled off in annotations
• [x] Hide annotations with no visible features/tags
• [x] Persist user-specific visibility settings across sessions
• [x] Persist settings per user and per project
• [x] Include settings in project export
• [x] Restore settings on project import