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

MISC: Get ride of VivyCli and everything is placed in VivyApplication (because of the way to get the log sink)

/!\ REGRESSION: We no longer have the line of the last lua error!

- The message no longer outlives the logger
- Update the objects that use the LOG API
- Print more things with the stderr dispatcher
- The UI, src/Lib, src/Lib/Ass, src/Lib/Document use the new LOG API

LOG: Add some logs, pb on flush on exit because the messages outlive the logger (string_view problem)

TODO: Don't depend on the order of the includes

LOG: Add a way to get the sink from the VivyApplication and add more possibilities for logging things

MISC: Separate implementation of the different classes of Log.hh into multiple namespace Vivy{} in the Log.cc file