Changed image selection markup in main window.
Separated directory parsing process into steps.

(Code will not run properly until controller changed to accommodate
ChoiceBox controls.)
Implemented ComboBox selection in place of ChoiceBox.
(Miscellaneous) Shortened listener.
Set expanded pane on app startup.
(Miscellaneous) Changed single-subtraction tool area styling.
Created label for displaying root directory.
Implemented tooltips for tool area controls.
Merge branch 'toolsetControls'