AppVeyor recently updated their Qt version to 5.11.2 which breaks
builds, because of the outdated path. Using this path should always
ensure that the newest subversion of Qt 5.11 gets used.
Move the AppVeyor install scripts to a separate script file. This
shortens the appveyor.yml file and makes it easier to introduce new
install scripts for other build environments.
There is one minor change to the install script, and that was to change
directory back to the obs-studio git root directory.
