Media manager: don't create empty namespaces FS#2642

Previously the media manager created an empty namespace whenever you
opened a non-existing namespace with upload permissions. Now the current
namespace is only displayed in the tree but not actually created.
Abort old quick search requests when starting new ones

This prevents old requests that need longer than the newer ones (likely,
as if you type more less results are returned) from overriding the
results list.