Added action to context menu--'Edit variables'. When this selection is made, STPlotWidget emits editVariablesClicked() signal. Thinking about moving the context menu from STPlotWidget to STWidget.
Connect statement added fielding STPlotWidget::editVariablesClicked() signal to STWidget::toEditVariables(). This brings up the STEditorDialog to edit the variables added so far. The temporary Edit button that appeared at the bottom of the widget window has been removed.