[kcms/lookandfeel] Support latte layout

- This patch enables LookNFeel KCM to load and
unload latte layouts on demand when a looknfeel package
is applied from the user. Designers can include
in their package a `layouts/looknfeel.layout.latte` textfile
if they want a specific layout from Latte. In such case
Latte is also enabling its autostart.
- In case the looknfeel package does not contain
the previous mentioned latte layout file then we
can safely assume that the user does not want Latte
at all and in such case Latte quits and is removed
from autostart.
- In order to apply or not the latte layout the user must
have chosen to update the desktop layout from the relevant
ui checkbox.
- This patch to work needs at least `Latte Dock >= 0.10.5`
- If the Latte requirement is not met then the user
will not observe any changes.