allow setting of text instead of an icon

Show/Hide text is less mental strain to understand and can be localised.

• Add two new attributes show_text and hide_text and corresponding java methods.
• If text set then use this in preference over drawable

Suggestion: treat the text as a drawable create inner class that extends Drawable. This way it can just be handled as a drawable and the logic of the view can stay the same (not if text do this). If possible get the typeface from the edittext in case a custom font has been set.