Bring QWE 5.15.11 from Debian unstable for Focal
This is, again, a fresh start of QtWebEngine packaging for UBports Focal. This still uses a multi-orig-tarball approach, but compared to Xenial, this should be cleaner for a few reasons:
- Now uses system (GCC) compiler instead of Clang.
- Built against more system components e.g. libicu, node-yargs
- No more Xenial compatibility patches.