ECMAddQtDesignerPlugin: pass code sample indirectly via variable name arg

Summary:
The initial API asked to have custom C++ code to be parsed as string
arguments. As any ";" in such strings would be treated as list separators,
the initial API as workaround required to encode those in the passed
code text with "@SEMICOLON@".

As alternative variant the approach from the GENERATE_EXPORT_HEADER()
macro and its CUSTOM_CONTENT_FROM_VARIABLE is now used, where instead
the name of the variable is passed, from which then the full code could
be fetched and directly used.
As API user one no longer can pass code snippets directly inline as
argument, but no longer needs to use the awkward @SEMICOLON@.

As ECMAddQtDesignerPlugin is not yet part of a released version,
the API can be still changed.

Reviewers: Frameworks, Build System, vkrause

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D23725