The pointer passing for display options is deprecated. This removes the
excess casting as the EDA_FRAME didn't need the base call with no value.
All requests for display options are now returned const and are updated
with a Set() routine after modification.
In Gerbview, this resolves an issue where the display options were not
stored because it was receiving the NULL from EDA_FRAME.
The board design settings file keeps our default values for various
thicknesses. We use this to replace a number of magic numbers with the
defined value to help self-document the code.
... and GetMsgPanelInfo.
Step 4 in the g_UserUnit eradication effort.
Also removes a couple of conversion routines that were close
enough to extinction.
(cherry picked from commit c75da51)