Unify selection of 'valid' Qt versions for welcome page

Change-Id: I79ba4ac9f132a535efd9d362790b8e1d2b62d900
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Move management of 'selected' Qt version to ExamplesListModel

Change-Id: Ia4c73a0e9985e968265d7698e3a0a3cc58dc7aaa
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Remove Qt 4 fallback for examples browser

Remove fallback file, and rather use the examples-manifest.xml,
demos-manifest.xml files that are part of Qt 4.8.0 or newer.

Change-Id: I6291bd01b75b891a5ddab7fb01cdfc29ed8bec34
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Examples: Remove loading of *.xml files from Qt demos/examples dir

Not sure what this code path did originally, but since at least
Qt 4.8.0 examples, demos manifest files are only in doc.

Change-Id: I6edf1f712fc06d290edf0fe978a6635966084e60
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>