MPV: Fix the MPV controls

See merge request !26

- Correct minimal version
- Correct message status

- Don't do anything if no MPV created
- By default don't create the MPV, let the user do it himself

Try to fix crashes when creating and deleting MPV

Closes #6 and #5

See merge request !23

MPV: Don't load the video by default, force the user to click on the button to avoid some strange race conditions

The race condition between Qt's widget creation and mpv getting the WID
doesn't seem to be fixed for now, will need to investigate further.

MPV: Use mpv_destroy to delete the player, don't use the "quit" command as it seems to be the way recommended in the client.h header

Try to force the window creation before passing the WID to MPV to fix
the possibility where the window is not already created and thus
everything crashes.

Add clean logs support + dependent MR

See merge request !21

Theme system

See merge request !22

We do that to move all the logic to switch between palettes and qss
files into the same class.

Add script functions to get VivyDocument informations

See merge request !24

We destroy the log console before the final flush, take that into
account => disable handle message for the ConsoleLogSinkDispatcher if
the corresponding LogConsole was destroyed.

This is a quick and dirty solution, but I don't know any other way for
now!

Because we create a shared_ptr, and because the MainWindow will delete
the widget on exit, we have a double free error...

LOG: Add a worker thread for logs, it will launch the `flush` operation in a thread-safe way when logs are available