Use JColorChooser to select colors of various letters

The ActionListener is unimplemented so the buttons do almost nothing. This
commit is to verify that the JDialog is shown correctly and that all
components are displayed at the appropriate position.
Implement appropriate actions for events when choosing color

However, the "initial color" rectangle does not change even when changing the
selected item in the JComboBox. A custom preview panel needs to be
implemented in order to achieve this.
Implement JColorChooser with custom preview pane

This preview pane allows the user to see how the text actually looks.
Update JavaDoc for ColorChooser class
Merge branch 'color_chooser' into prototype

Implementation of ColorChooser class is finished.