Work on #213 : Rename interface, add methods to get last selection.

PositionlessSelection renamed to SelectionArea because it isn't a subtype of
selection. Add javadoc to this class to explain what it is for.

Renamed getLastActiveSelection methods to getLastActiveSelectionArea.

Added getLastActiveSelection method to retrieve actual Selection instance.
Work on #213: add "last selection boundary" marks.
Work on #213: save / restore selection through editor adaptor.
Work on #213: implement `gv` command.

Extended Selection interface to know which mode created it and hence
which mode it should be displayed in. This could have been done using
an if/else if structure with instanceof checks, but this looked prettier.

Changed normal mode to add gv binding to new RestoreSelectionCommand.
Fix #213: Implement `gi` / resume editing feature.

Fixed problem with "last insert mark" - it had a off-by-one error.
Merge branch 'restore-selection' into up-master to fix #213.