Added support for button label colors

Added support for button label colors. Now text color can be defined in
the XML file as: color='value'.
Value can be: #RRGGBB #AARRGGBB 'red', 'blue', 'green', 'black',
'white', 'gray', 'cyan', 'magenta', 'yellow', 'lightgray', 'darkgray',
'grey', 'lightgrey', 'darkgrey', 'aqua', 'fuschia', 'lime', 'maroon',
'navy', 'olive', 'purple', 'silver', 'teal'