DigitDetector used by canvas mouse handler
Automatically add auto-detected digit to selected group in editmode
Drag corner of selected digit
View edit mode (drag image)
Toolbar buttons indicate current mouse mode
Prevent automatically adding digits to dot groups
Draw auto-detect region + auto select group on addition to tree
Show corners of selected digit
Don't show selection rectangle when dragging a digit corner
Show selected object in tree (underline) + autocollapse objects that are not in the selection
Fix: don't show selection rectangle when dragging a digit corner
Fix rearranging order of root tree nodes