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)

- 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 a way to get the sink from the VivyApplication and add more possibilities for logging things

BUILD: Only place external headers in the precompiled header to avoid recompiling it every time a local header is changed

Also fix all the warnings present with clang and get ride of most of the
forward definitions when possible.

And some macros to get the current VivyDocument or current
ScriptDocument

UI: Use a custom icon provider for the QFileDialog for cases when the system is light themed and Vivy is dark themed and the reverse

Two main folders: Lib and UI. The Lib is specialized in Document and
Ass, the UI is specialized for private DocumentViews (private document
views) and others (future ScriptViews, etc)

BUILD: Use precompiled headers, store all forward declarations in the "Utils.hh" header and use namespaces to isolate vivy's code

MISC: Place all the vivy application in its own namespace, use sub-namespaces for the Ass and Utils parts

Adding vivyApp macro to access the application globally