libdeng2|FileSystem: Creating subfolder when parent has multiple feeds

When inheriting feeds, find the first one that actually returns a
valid sub-feed (i.e., the subfolder must exist in the context of
the parent folder's feed).

Also added a flag that disables the automatic population of new folders,
useful internally for folders created during the population process.
libgui|GLState: Blending enabled in default state

Complies with existing convention in the client.