ValueRangeWidget changes:
- Don't differentiate between type limits and machine limits
- Don't limit max value to dtype.max-1
- Added setBlank() which sets the widget display to '--'
Added DataExportOptionsDlg and associated widgets.
Added SubregionRoiWidget
Data Export: Use the new subregion widget.
SlotMetaInfoDisplayWidget: Handle non-ready slot.
Hdf5ExportFileOptionsWidget: Fix extension filter.
DataExportOptionsDlg: Cleanups.
MultiStepProgressDialog: Hide OK until progress is complete, and add a 'failed' display.
layer context menu: Use the new export ui.
layerstack: Call layer.clean_up() in all cases when a layer could be removed (not just for deleteSelected())
LazyflowSource: Keep a list of operators to cleanUp when the source is cleaned.
DataExportOptionsDlg: Init ui with model op's current settings for roi, normalization, dtype, axis order
format-specific file option widgets: Allow user to edit the path by hand.
Multiformat file options widget: Init format selection according to the operator's initial state.
SubregionRoiWidget: Init according to operator's initial state.
ValueRangeWidget: Ignore a transient overflow error when returning the current values.
layercontextmenu: Export layer: Clean up temporary operators after export.
(Also, deleted debugging code assertion.  Whoops.)