introduce the Defaults group for panel values

Summary:
when plasma is starting up, there are no panel views yet
this means that if a thickness (or other values)
are written to the config file and then the resolution changes before
the panel view gets created, we'll have a view that will default
to 30 pixels regardless what the layout js file specified.

This makes the scripting only write to a [Default] group for resolution
dependent properties when panels are not actually present
when they are already there, interact directly with the view which is
the safest option.

the view will use whatever is in the defaults group as default if the
resolution dependent is there, and when a new value is set for a
resolution dependent one, also update the default, which will
be used when screen resolution changes, as new default value

Test Plan:
* With virtualbox, change resolution continuously when plasma is starting up
with an empty setup, panel is the expected size now, as opposed to 30
* Stop Plasma. Change Screen resoluton with plasma *not* running, to a resoluton
still "unknown" to plasmashellrc, then start plasma again. Now the thickness saved
in [Default] is picked, instead of the arbitrary 30

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22893