desktopview: Add Plasma 6 pre-release versions to preview banner

Oliver Beardrequested to merge
work/olib/preview-banner-6 into master

Fixes #98 (closed)

Hardcodes Plasma 6 pre-release versions in the preview banner. Development versions are reported as "6.0 Dev", and with support for our upcoming "Alpha", "Beta 1", "Beta 2, "RC1" and "RC2" releases.

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