qt: do not take snapshots from the UI thread

Snapshots are performed via a call to:

var_TriggerCallback("video-snapshot")

causing its callback (SnapshotCallback) to be called synchronously, executing the following steps:

• wait for the vout thread to actually capture the (next) frame;
• encode the picture to PNG;
• write the result to disk (I/O).

Since var_TriggerCallback("video-snapshot") is called from the UI thread, all these blocking actions are also performed on the UI thread.

Move the call to a separate thread.

changed milestone to %4.0

added Component::Interface: Qt label

Oh, but this is just for the Qt part, when a snapshot is created from a hotkey (Shift+s on Linux), it's still called from the main thread. Maybe the asynchronous call to request a snapshot should be implemented in the core player? (what do you think @tguillem?)

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• 7d80a102 - qt: do not take snapshots from the UI thread

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• 7923e092 - qt: do not take snapshots from the UI thread

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• f7a767a6 - hotkeys: do not take snapshots from the UI thread

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I added a commit to also call the snapshot asynchronously from hotkeys (Shift+s).

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added 46 commits

• f7a767a6...e2a10a5c - 44 commits from branch videolan:master
• 1aa5ca65 - qt: do not take snapshots from the UI thread
• b3ccc7b0 - hotkeys: do not take snapshots from the UI thread

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