[Static] [Plasma] 421.160601.0 fixes theming issues

[S.W.B.]

Hello everyone,

I'm releasing a new version of the GeckoLinux Plasma edition to fix some
major theming issues that were still present in prior versions.

It turns out that it is not at all easy to create a unified visual theme
for a heterogeneous set of applications that are coded with GTK2, GTK3,
Qt4, and QT5, plus the additional complication of LibreOffice, which can
use multiple different backends for its interface. In previous editions
of GeckoLinux, I was using a theming hack via the
`QT_STYLE_OVERRIDE=gtk` environment variable in `/etc/environment` to
force all QT programs to use the GTK Numix style. However, this caused
bugs such as disappearing filenames when selected in Dolphin, and it
also prevented users from changing away from the default GeckoLinux
theming. It is recommended that users of older GeckoLinux users remove
the aforementioned environment variable.

In this new 421.160601.0 Plasma edition of GeckoLinux, I am now using
the Numix color scheme with the native Qt5 "Fusion" theme, while the few
remaining Qt4 apps seem to behave better with rendering from the GTK
Numix theme. The GTK2 and GTK3 programs are still using native Numix
theming, and the `SAL_USE_VCLPLUGIN=gtk` environment variable is now set
in `/etc/environment` to force LibreOffice to use the GTK2 Numix theme,
which is still the best looking and least buggy option at present for
LibreOffice. GeckoLinux users can also now modify the theming to their
own preferences, and it will not longer revert to the old theme.

Additionally, it appears that the Kwin 5.6 compositor is now stable
enough to enable by default. This enables some nice visual effects, and
makes for a smoother interface. However, users with weak systems or
buggy graphics drivers may still want to search for "Compositor" in the
applications menu and disable starting the compositor.

Additionally, the nasty warning messages about broken signatures from
the Google add-on repository should be eliminated now.

Here's a screenshot of this edition of GeckoLinux Plasma:
https://cloud.githubusercontent.com/assets/2692138/15724397/7e8f9144-280c-11e6-9a2e-2bf6d3e761bc.png

And you can download it here:
https://susestudio.com/download/d9c0e5a5e632a33705aed433b661def8/GeckoLinux_Plasma_Stable.x86_64-421.160601.0.iso

Thanks for reading, and have fun!