Fixed goBack() and added new functionality

goBack() functionality was broken. Fixed it.
Added setBackButton() method to HeaderPanel.java. setLeftButton() is hard-coded to the string 'BACK', which precludes setting a back button with text such as 'Cancel'.
Added methods to TextBoxBase.java:
getMaxLength(), getVisibleLength(), setMaxLength(), setVisibleLength(), setPlaceholder()
Exposed 2 methods in TouchPanel.java: startTouch() and stopTouch().
This is needed in places where you have a map embedded in StackPanel, etc.
Without this, you cannot pinch and zoom the map.