Fixed goBack() bug and added features

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.

Signed-off-by: Frank Mena <frank.mena@noledgy.com>