client/util-selection

Update the selection with a given group or interval or, if no group is given, clear the selection.

For categorial selections the following rules are used:

1. none selected: add the group to the selection
2. one selected and the group is the same: invert the selection
3. one selected and the group is different: add the group to the selection
4. more than one selected and the group is in the selection: remove the group from the selection
5. more than one selected and the group is not in the selection: add the group to the selection

For continuous selections the following rules are used:

1. no range selected set the range equal to that of the group
2. a range selected and the group is outside the selection: extend the selection to include the group
3. a range selected and the group is inside the selection: set the endpoint closest to the group to that of the group