[faces clock] bare-bones switching mechanism

During the Plasma sprint, we talked about that we're still not quite
happy about code duplication in the clocks available for Plasma, and the
complexity of the digital clocks code, which makes it very hard to get
the rendering juuuuuuuust perfect, because it still isn't in many cases.

So, new approach we came up with:
- Clock faces are different representations (done in QML) with a shared
underlying mechanism. This resembles a little bit what we did with
libplasmaclock during 4.x times, where we shared complex code for
timezone handling, etc. in a shared library. This approach is different
in that the clock actually gets its own (non-binary) plugins in the form
of easy to write clock faces that are geared and perfected towards a
specific scenario.

The idea is to use a Loader and load a clock face dynamically during
run-time, instead of loading the "fixed" DigitalClock.qml directly.

- Move DigitalClock.qml under faces/ directory
- Create a really simple alternative face
- Allow switching from the config ui

Rough, first prototype, in particular:

- rendering of the previews is somewhat very broken
- the exposed api is pretty much main.qml
- zero polish

Further steps:
- flesh out API to present to clock faces, perhaps as a base QtQuick
  class to implement the clock faces, expose API in a less ad-hoc way than
  right now
- design mechanisms to avoid the complexity requird by the vast matrix
  of rendering / constraint scenarios, perhaps splitting up the faces
  further into Horizontal.qml or Vertical.qml

All needs a lot more fleshing out, here are some ideas where to take
this branch:

- Use packages for the clock faces, allow installing third party
  clocks, hello store.kde.org
- Not just digital clock faces, but also analog; Kai has a prototype to
  change just the theme of the clock, take ideas from that
- Perhaps start easy with just a bunch of different pre-defined faces,
  and allow 3rd party faces later on, this way we can get it into the
  hands of users a bit earlier on and learn about mistakes through
  feedback early on
- Plasma is still all about clocks.

I'll hack a bit further on this, but since notmart asked during our
meeting this morning, I thought why not share at this early point,
perhaps others find it interesting as well.

Personal Note: I worked on the very first digital clock in Plasma, way
before 4.0. The plan was to simplify a very artistic experimental train
clock done by Ruphy into a basic digital clock, which we ended shipping
with 4.0. I wanted to use a 2 hour train ride (ironically) from Cologne
to Nijmegen to get that work done. That was in 2007. It'll be ten years
this year that I make the mistake of starting to hack on our clocks'
code.

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