Folder sharing and search over folder shares

Nobody has claimed this yet.

People would be able to share files by sharing entire folders and would be able to search files over peoples shares. [Direct Connect protocol](https://en.wikipedia.org/wiki/Direct_Connect_(protocol)) has this ability but why not to include it into Matrix?

Example use case: A group of travellers create Matrix room to communicate with each others. During their travel everybody takes a lot of photos. As people want to share their photos they can use Matrix client to share photo folders and everyone download the photos from the shared folders.

In advanced versions the clients could make thumbnails from shared photos and users can just click thumbnails of the photos they want to download.

Some file standard should be developed to handle file indexes of shared folders. Then search function just searches over these index files. [DC++](http://dcplusplus.sourceforge.net/) is popular open source client that already does this. Perhaps some ideas for implementation could be taken from DC++ source code. Why not also make the implementation Direct Connect compatible? Then we could have Matrix-Direct Connect bridges.

No repository files or tests are identified in the issue. Start by reviewing the Matrix specification and the linked Direct Connect and DC++ references; done would require a decided, specified approach for shared folders, file indexes, searching, thumbnails, and any bridge compatibility.

Written by the indexing model from the issue text.