Fixes #7 - Unfitted background plotted.
Fixes #7 - Unfitted background plotted.

When a background was selected, loaded and then unselected,
the fit was updated and selected once more without update,
the background was plotted when not fitted. Therefore the
variables handles.bg_use and handles.bg_use_flags were
renamed to handles.bg_use_loaded and
handles.bg_use_loaded_flags and new variables
handles.bg_use_fitted and handles.bg_use_fitted_flags
were introduced. These new variables represent the state of
being fitted. Each background also got fitted member
variable.

lumik/bgcor