qTox1.17.3

Features:

• osx: Add support for macOS 10.16, remove support for macOS 10.13

Bug Fixes:

• Build: cache debug deps during 2nd stage

Osx:

• Allow rebuild using build script
• Fix missing variables in macOS build script
• Update deprecated QDateTime and QProcess APIs
• Don't tap kde-mac/kde since it us deprecated and unused

Tools:

• Use correct hash when automatically updating flatpak version
• Use a subdirectory for release source archives
• Travis: Install virtualenv for nightly build upload script