Allow the user to manually select a mirror

This feature is needed because:

• The automatic mirror selection algorithm sometimes picks a bad option.
  • (Just 2 weeks ago) After running overnight on the default mirror, the installer only downloaded 12%. The user reported that picking a different mirror (using the Qt SDK Repo Chooser [1]) sped things up dramatically: https://forum.qt.io/topic/92445/

• • Geographical proximity does not imply mirror quality.
    • A user from China found Chinese mirrors too slow, and opts for non-Chinese mirrors instead: http://lists.qt-project.org/pipermail/interest/2013-December/010336.html
    • A user from China requested the removal of a Chinese mirror: https://bugreports.qt.io/browse/QTWEBSITE-677

• • The algorithm doesn't pick the closest mirror anyway.
    • A user from Israel was given a Japanese mirror instead of a European mirror: https://forum.qt.io/topic/87221/
    • A user from Cyprus was given a Chinese mirror instead of a European mirror: https://bugreports.qt.io/browse/QTWEBSITE-637

• A specific mirror might sometimes be blocked client-side: QTIFW-1079

[1] https://github.com/JKSH/QtSdkRepoChooser/