This fixes some const qualifier violation in modules.

This was a bit too early.

This reverts commit b827afa2.

Conflicts:

	configure.ac

This reverts commit 66341140 and correctly undefine out: if needed.

Reported by Hugo Beauzée-Luyssen.

Warnings that are 'safe' to be treated as error should be added here as well.

I hope this will raise VLC code quality, and tend to lower the hudge number of warnings. Some of them are sometimes letal but hidden by others.

When using tree playlist mode and moving items within sub-nodes, the nodes
will now stay open.