Ability to change files directory without need in TDLib restart

[vkryl]

Something like SetFilesDirectory (bool move_files). I'm not sure if there'd be any need in such method, if TDLib would have had an ability to set path for each DownloadFile (#379).

This would allow clients to implement download path setting without need in TDLib restart (which may be quite tricky in the UI) & without need in manually moving each folder, if there's such need (clients may not be aware of new or existing TDLib folders used inside filesDirectory).

[levlam]

It is hard to move all files and it is especially hard to do it when TDLib is run. For example, moving all media from one drive to another can take minutes or hours. Also all file paths known to the client will become invalid, so the app will fail to open any file just after file has been moved and before updateFile is received. There can be also numerous issues with already opened files.

[Grod18rus]

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