platform-demos: Add link to GMenus in GApplication.py.page
platform-demos: Add note in image.py.page

Add note about including imports and instaliazing application
before trying to run the code alternate code snippet.
platform-demos: Reflow, edit, punctuate, reword image.py.page

Notable deletion: removal of instructions for using stock
icons.
platform-demos: Rephrase line about string encoding in Python 3.x
platform-demos: Add e:hi on alternative code snippet

To improve readiability and visually differentiate between code
snippets, added e:hi highlight. Highlighted code that is
significantly different in the alternate code snippet as
compared to the main snippet.

Add note explaining why some code has been highligted.
platform-demos: Add note style tip to work with Strings
platform-demos: Reword introduction about Properties