qmlls: make only -B required and use .qmlls.ini from build folder instead

Qmlls requires command-line parameters for working correctly, currently:

• build folder
• import paths
• documentation directory

Technically, if qmlls has the build folder, then the import path and the documentation directory can be found from the build folder instead of the source folder.

Clean up the .qmlls.ini generation to:

• generate .qmlls.ini inside the build folder with information about import paths and documentation directory (and don't set no-cmake-calls or other user settings there)
• generate the .qmlls.ini inconditionally (and keep the .qmlls.ini source folder generation behind the QT_QML_GENERATE_QMLLS_INI variable)
• Use the source-to-build folder mapping in qmlls to load the .qmlls.ini from the build folder, before loading the settings from the sourcefolder (source folder settings overrides the build folder settings)

This should make the qmlls integration into clients easier as qmlls now only requires one argument, the build folder.