[FEATURE] allow multiple folders for static datastructures

[J. Peter M. Schuler]

A templavoila datastructure is tightly coupled with it's template. Although one could argue, that most templates might be "main content" and "sidebar" content, this doesn't need to be the case and if merging two systems, although the structure might be the same, the naming of the fields might not be. Additionally a datasctructure might also support really complex options and fields. Thus a datastructure and template combination might be called unique. Of course for "one design" an administrator will probably make a combined datastructure for all templates.

Nevertheless a TYPO3 instance can hold multiple designs (e.g. per domain). As it is best practice nowadays to define a web template as an extension this means that the static datastructures should be supplied in the very same extension.

This leads to the necessity to define more than one folder in extension configuration, e.g. multiple paths separated by a semicolon (colon perhaps breaks volume-path-separator on Windows), as using a fileadmin/templates folder doesn't make sense then.