Feature: Styling: add more flexibility

[bytesparrow]

To style the field, a function
render_customfield_core_course_course_thumbnailimage needs to be added.

To be more flexible (e.g. have field-names in variables, not in code), the last part should be optional so that
function render_customfield_core_course_course is ok.
The property "fieldshortname" will be added to the model in that case.

The "oldfashioned" way to name the renderfunction will not be removed, the "new" will be an alternative.
What do you think about that?