Small fix to remove_background() change
Moved range selector left-right switch to widget

Previously suggested switching left and right in
BackgroundRemoval.bg_to_plot(). Here changed to doing it in
ModifiableSpanSelector, by changing which edge is moved when crossing
over.
Made ModifiableSpanSelector edge switch strict

Fixed imprecision in edge switch in ModifiableSpanSelector when the
change in position was large, i.e. it now precisely keeps the position
of the edge being passed over.
Option for modal interactive dialogs

Added options for interactive GUI tools to be modal, allowing e.g. for
scripting general routines.