Move getter method naming to BindingMapping

Move getter method naming to BindingMapping

Both BaseTemplate.isAccessor() and a chunk of
AbstractTypeGenerator.getterMethodName() should actually be held
in BindingMapping, so we retain consistent naming of things.

This patch introduces the constants necessary for this to work
and migrates the code there. This cascades to various other places,
like mdsal-binding-dom-codec, where we extract copy&pasted code
to a common utility class.

JIRA: MDSAL-18
Change-Id: I5fb285f00658ce81f60890d03844b51e19bd4a40
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>