Qt5127 (#489)
* update to Qt 5.12.7
switch to online Qt installer.
Note the latest Qt online installer, 3.2.1, can crash on macos.
https://bugreports.qt.io/projects/QTIFW/issues/QTIFW-1592
For this reason we use the previous installer on macos.
* add missing online install script.
* update more travis install scripts.
* also switch Qt 5.9 builds to 5.9.9.
make install target optional in install-qt-online.
* fix linux install scripts, enable Qt archive
* add new installer dependencies.
* fix travis linux install scripts some more.