quick annotation of a structure via sequence features (that can be undone)

a common activity in Jalview is to mark up regions of a structure by highlighting positions on the sequence. Sequence features are great for this, but it is easy to mistakenly annotate the wrong position.

We need:
* undoable sequence feature creation
* bulk editing of features (ie select area and right click->edit/remove sequence features...)
* ability to hover over a region on structure and create a feature in one key stroke (rather than at least two.. B, then menu->... then OK to create the feature)