CMake: reduce Qt dependency back to 5.9

- Although KDE CI onlt tests with Qt 5.10, and KPMCore 4 requires
   Qt 5.10, Calamares is still ok with older Qt and KPMCore 3.3,
   so drop the dependency back down again. This means, though, that
   the code will build against a Qt version we don't usually test.
   We're going to assume that Someone Else does the LTS-Qt testing
   for us.

CMakeLists.txt

@@ -116,7 +116,7 @@ set( _tx_bad be eo fa fr_CH gu kk kn lo mk ne_NP ur uz )
 ### Required versions
 #
 # See DEPENDENCIES section below.
-set( QT_VERSION 5.10.0 )
+set( QT_VERSION 5.9.0 )
 set( YAMLCPP_VERSION 0.5.1 )
 set( ECM_VERSION 5.18 )
 set( PYTHONLIBS_VERSION 3.3 )