mock: add subitems/node support

Example:

"mock://node[2]{video_track_count=1};node_count=4;length=1000000;node[1]{length=2000000}"

Will create the following sub items:

• input_item_New(mock://length=1000000, submock[0]);
• input_item_New(mock://length=2000000, submock[1]);
• input_item_New(mock://video_track_count=1, submock[2]);
• input_item_New(mock://length=1000000, submock[3]);

If specified with node[]{}, a sub item will use the params between "{}". If not specified, all subitems will use the same params from the url (here: "length=1000000").

This will be needed for the future and reworked media_list_player test.