kbd preview: don't fall over unfortunate arg combos

as it turns out it's fairly easy to fail the original xkb assert when
using variants with certain models that don't actually sport support for
the variants

e.g. `--model applealu_iso --layout gb --variant mac_intl`

to deal with this we no longer assume that geometry compilation will
actually work and instead switch the qml ui into an error state when
problems appear. furthermore we'll run the arg combination through
setxkbmap|xkbcomp to get a sense of why the server might have failed to
compile the geometry and use that as detailed error output

I need to point out that the preview failing doesn't necessarily mean
the layout application as a whole will because there's different code
paths the kcm and the kded will take to apply layouts so depending on
which path is taken the configuration may be partially applied or not at
all (e.g. with the scenario from above the end result may be that model
doesn't get applied but the layout and variant will)

since these changes required some rejiggering of the pointers and their
life times this has also seen some related cleanup. notably the geometry
is now in charge of the life time of the root xkb object and the
previous qsharedpointer cleanup (which was really scoped pointer cleanup
but scoped pointer has kinda sad api for custom deleters) is now instead
a unique_ptr with custom deleter.