Set a default font DPI if not set

Summary:
Currently X and Xwayland (unless set otherwise) defaults to a DPI of 96.

GTK on wayland (unless set otherwise) defaults to a DPI of 96.

Qt (currently) defaults to physical size/resolution. This leads to
reports that Wayland looks big.

Qt is potentially also changing to 96 for > Qt5.10.
(it's in review at the time of writing)

Now kwin has proper scaling support, legacy apps get scaled by the
compositor without font hacks, and apps that scale themselves need a
non-scaled font DPI. (i.e a 4k screen at 2x still wants a DPI of 96 not
184).

We want to change the wayland scale factor not the font.

If a user does override this setting that override will still work.

BUG: 374978

Test Plan:
don't have this setting explcitly set
dolphin --platform wayland and dolphin --platform xcb have everything
the same size. Even on a scaled display where one is scaled up by the
compositor and one is native.

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7185