Syncplay 1.6.4

What's new in this version:

- Add TLS (SSL) support in the Windows binaries. Opportunistic TLS is now supported on all platforms (included in PR #231 by albertosottile, requested in #217 by alxpettit)
- Add support for releasing via Snap and AppImage. Travis CI now automatically builds releases for macOS, Snap, and AppImage after every commit. This version of Syncplay is also published in the Snap store and on AppImageHub (Snap: PR #231 by albertosottile, requested in #199 by Mikaela; AppImage: coded by albertosottile)
- Add support for installing Syncplay using setuptools via a setup.py script. This required a change in our folder structure, though this change should not affect end-users. (PR #231 by albertosottile, contributions by Et0h and ObserverOfTime via PR #230, suggested in #207 by alxpettit)
- Remove the "Update list" button for public servers from the configuration dialog (PR #233 by Et0h)
- Fix an issue that prevented to use SMPlayer (issue #234 raised by elvissteinjr and resolved by Et0h)
- Some images in the GUI now scale their pixel density for Hi-DPI/Retina displays (819e6b6 and 8f5b77c by albertosottile)
- Add getValueForOS function in constants to store and extract different values for different operating systems (ff6bb74 by albertosottile)
- All the png files in resources are now automatically included in the distribution bundles (3a4cf41 by albertosottile)
- Do not ask anymore to "Press enter to exit" after closing mpv when Syncplay is used from its CLI (issue #238 raised by alxpettit and resolved by albertosottile)
- Embed darkdetect (GitHub, BSD-3-Clause) version 0.1.1 in the project vendor folder
- Some files in the repository are now released under the MIT license, while the repository stays on Apache 2.0. This is explicitly stated in the header of those files. At this moment, this exception applies to README.md and travis/appimage-script.sh.
- Upgrade qt5reactor to version 0.5
- Upgrade py2exe to version 0.9.3.0
- Upgrade PySide2 to version 5.12.3