eglfs: use QGenericUnixTheme if requested

We need to have reasonable paths for loading icons and such things, just
as in any other Unix GUI environment.  However there is some concern that
it would be too much of a behavior change if there was a theme by default,
so for now it's required to set the env var: QT_QPA_PLATFORMTHEME=generic
That works because QGuiApplicationPrivate::createPlatformIntegration() reads
the env variable and passes platformThemeName to init_platform().
Step 3 in init_platform() does not find a theme plugin by that name
(because QGenericUnixTheme is statically linked via libQt5PlatformSupport.a).
Then in step 4 it iterates the given platformThemeName plus any which
were returned from QPlatformIntegration::themeNames() (which in our case
will be an empty list) and calls createPlatformTheme() with each of
those, until something is returned.  So,
QEglFSIntegration::createPlatformTheme() will be called with the
value of the QT_QPA_PLATFORMTHEME env var, and
QGenericUnixTheme::createUnixTheme() will create the generic, KDE or Gnome
theme depending on that value.

Change-Id: Id16b881819ba872830b019ab147b32fbc2156520
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

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