implement a panel to add missing tags

We have to provide a new functionality which enables the user to add missing tags to the target segment content. This can be achieved by listing all missing tags in the target column (compared to the source column) in a new panel.
The tags can be added by click or keyboard short cut to the caret position in the segment editor (like MQM tag adding).
If a tag is getting used, it disappears with a nice fade out effect from the list.
The shortcut can be something like CTRL - T and then a DIGIT. If browser tab creation can be stopped. CTRL-T (or in general the first part of the key command) opens the tag panel.
The numbers / digits are getting reordered / reused if a tag disappears from the list when it is getting in use.
This new "add tag panel" should be added as new accordion panel in the right sidebar, between meta panel and comment panel.
The "open comment panel" functionality must be checked / fixed.