IO priority option/Process mode background

[ilyvasile]

Suggestion

Adding IO priority/Background mode of process to the advanced options (similar to process memory priority).

Use case

When IO throughput of storage is low, and active seeding, there's perfomance issues in other applications that reading high bitrate content from the same storage. With IO priority of process setted to low, there will be no need to manually reduce upload speed, OS will automatically delay IO reads for qBT when other IO intensive applications active on the same storage.

Extra info/examples/attachments

Some API info about implementing this available at https://superuser.com/a/226419 and docs.microsoft.com
Currently using process explorer menu to achieve this on Windows.

[FranciscoPombal]

Note: this is not the same as the existing memory priority advanced setting:

qBittorrent/src/gui/advancedsettings.cpp

Line 413 in cd36359

#if defined(Q_OS_WIN)