Allow setting name_prefix project-wide

Instead of setting it for each individual library in a project, which will end up being very verbose for projects like gstreamer plugins, it would be nice to be able to set it as a project option, maybe prefixed with "library_".

Individual libraries could override the project-wide setting by passing "name_prefix" as currently possible.

No files or tests are named. Start by locating the existing library-level name_prefix handling and the project option definitions, then trace how library options inherit or override project settings. Done means a project-wide prefix is supported while an individual library can still override it.

Written by the indexing model from the issue text.