Add support to multi select using modifier keys

Fix #1211: Multiselect only if Ctrl-Key is pressed

Add support to only allow multi select when Ctrl-Key, Shift-Key or
Meta-Key (Cmd-Key on Mac) is used when clicked. Tutorial updated to
allow testing of this feature
Merge pull request #1711 from mathiasdose/feature1211

Issue #1211 Multi select when using Ctrl-Key and Shift-Key