Implement loading feature to all panels

Feature Requests

Description

current status:

• we have loading for text and annotation panel
• we need loading for all panels (metadata, image and tree/content)
• only the whole panel needs loading independent from content and possible tabs

how it should be:

• first empty state, then loading, then text (independent in each panel)

User Stories

As (role of the user) I need (what has to be done) in order to (what I want to achieve with this feature).

Classification

Is this feature an enhancement of existing code or a completely new feature?

• enhancement
• new feature

Related Tickets

Add all related issues.

/cc Mathias Göbel, Frank Schneider, Michelle Weidling