Review Bar: Refactor saving

[janiswehen]

For clearance: hasChanges means that either one layer of a group has some changes or the metadata of the group has changed. When we save changes, we always want to save the layer data and the metadata.

• the save button in the review bar asynchronously saves the current annotation group.
• the save button is greyed out if the current annotation group has no changes
• the save popup is not used but is still in the code base, so we can use it in the future for save asand duplicate.
• if the exit button is pressed and the user has changes to any annotation group, he will get prompted if he wants to save all or discard changes.
• the next and previous button of the review bar has a pop-up, which warns you if the current document is not saved on the hard disk (Review Bar: Next / Previous Image buttons #535). Currently the popup is always opened. It should only be opened if the active document is unsaved.
• the save button on the top right is removed
• Annotation Groups show an indicator if they have unsaved changes

Dependencies

• Review Bar: Swap between annotation groups #537

Ref: #515