Add 'ANDROID_GUI' target property to mark Android applications

Also add a 'CMAKE_ANDROID_GUI' variable to set the property default
so a project can easily make all executables Android applications.
An Android application executable file has the same extension as a
shared library (.so).

Help/variable/CMAKE_ANDROID_GUI.rst

@@ -0,0 +1,5 @@
+CMAKE_ANDROID_GUI
+-----------------
+
+Default value for the :prop_tgt:`ANDROID_GUI` target property of
+executables.  See that target property for additional information.