Rework virtual function page in ProgrammingGuide

[JBludau]

In the wake of #529 and #530 some inconsistencies in this page became apparent. Thus this PR proposes:

• Renaming the classes in the example Base and Derived and the virtual function Bar as in the corresponding images.
• changing the wording when describing a call to new from initializing to constructing.
• Filling in some text for readability.
• corrected the problems with the images identified in #530. -> redrawn with fixes to the stated issues and linking them to the text

[JBludau]

Nice, this time git seems to show the .png images in the 'files changed' view