Show and use versioning of segments in the GUI

A segment history will be implemented in translate5:

• The history shows all previous revisions of a segment of the same translate5-task including all segment metadata and including the initially imported version
• It is shown in a modale pop-up window.
• As first entry of the Grid in the pop-up-window the current segment version is shown, as second the previous one, as third the one before the previous, etc.
• By choosing a revision, the revision is taken over as current segment content
• For revisions, that are taken over, the translate5 metadata "last editor" will be the person, who did take over the revision.
• For revisions, that are taken over, a new translate5 autostatus "restored from history" will be introduced.
• the history will only show the history entries of the current target alternative
• Taking over a history entry is possible via Keyboard-Shortcut and via Mouse.