core,tracker: Fix upload of files

core,tracker: Fix upload of files

core:
MediaContainer: Add empty children counter. Add check_search_expression
function, which set create_mode on true, when "upnp:createClass" operand
was detected.
SearchableContainer: Use check_search_expression in simple_search function
to determine child counter.
SimpleContainer: Use check_search_expression to search both in empty and
non-empty children, during adding new item.

tests: searchable-container-test: Add children counter to MediaContainer
class. Add empty function check_search_expression in class MediaContainer.

Previously it was impossible to add new item, when matched container was
empty. For example user can't add picture, when don't have any picture.
Now it's possible, because now search function return also empty children
container, when search expression contains "upnp:createClass" operand. This
works when HttpPost was used to upload file.

https://bugzilla.gnome.org/show_bug.cgi?id=660885