Introduce LLView::isAvailable() to test enabled & visible.

Better term? The point of the method is to verify that a user could
actually interact with the LLView in question.

indra/llui/llview.cpp

@@ -438,6 +438,18 @@ void LLView::setEnabled(BOOL enabled)
 	mEnabled = enabled;
 }
 
+//virtual
+bool LLView::isAvailable() const
+{
+    return isInEnabledChain() && isInVisibleChain();
+}
+
+//static
+bool LLView::isAvailable(const LLView* view)
+{
+    return view && view->isAvailable();
+}
+
 //virtual
 BOOL LLView::setLabelArg( const std::string& key, const LLStringExplicit& text )
 {

indra/llui/llview.h

@@ -304,6 +304,11 @@ class LLView : public LLMouseHandler, public LLMortician, public LLFocusableElem
 	BOOL			getVisible() const			{ return mVisible; }
 	virtual void	setEnabled(BOOL enabled);
 	BOOL			getEnabled() const			{ return mEnabled; }
+	/// 'available' in this context means 'visible and enabled': in other
+	/// words, can a user actually interact with this?
+	virtual bool	isAvailable() const;
+	/// The static isAvailable() tests an LLView* that could be NULL.
+	static bool		isAvailable(const LLView* view);
 	U8              getSoundFlags() const       { return mSoundFlags; }
 
 	virtual BOOL	setLabelArg( const std::string& key, const LLStringExplicit& text );