On-demand refresing of external volumes

[harri]

Current external volume indexing method is not convenient when size of volumes grows big since the whole volume must be refreshed after every change. Better method would be e.g. the following:

• when user browses to a folder that belongs to an external volume, that folder is reindexed immediately
• Photovault could listen to file change events (if operating system provides such) and index files immediately.
• Otherwise Photovault could do every now and then a full reindexing in low-priority background thread.

Technical issues:

• Currently it is difficult to determine if a folder is representing a directory in external volume. Probably schema change is needed.
• Now indexing proceeds in recursive depth-first search. More fine-grained control of traversal order is needed
• Probably some kind of priority based scheduling is needed so that the folder that user has just selected would be indexed ASAP. Still memory consumption must be kept in control (i.e. no several concurrent indexing tasks with different priorities...)