Video playback with mpv

Add video playback with mpv

• Add the mpv widget
• Enable the widget only for video able documents
• Control the widget with a small set of buttons
• Seek mpv with slider that indicates the current position in the video
• Use std::chrono in the MpvControls to be sure to have the correct time unit...

Not mpv related changes

• Don't del and add docks if they are already present, just swap the contained widget => don't reset the position of the dock at sub-document updates

What it does not for now :

• Get rid of controls on MPV view when the dock goes fullscreen

src/Lib/Utils.hh

 #ifndef VIVY_UTILS_H
 #define VIVY_UTILS_H
 
+#ifndef __cplusplus
+#error "This is a C++ header"
+#endif
+
 #include <QString>
 #include <QFileInfo>
 #include <QStringList>
@@ -9,6 +13,10 @@
 #include <QJsonDocument>
 #include <QtGlobal>
 #include <type_traits>
+#include <chrono>
+
+// Use chrono instead of std::chrono...
+namespace chrono = std::chrono;
 
 // Prety define for OpenMP's parallel for loop with indentation not fucked up
 // by clang-format.