Remove 2  global variables: g_DrawBgColor and g_ShowPageLimits.
They are now member of EDA_DRAW_FRAME.
This change could fix Bug #1330781 (Background color change in Eeschema affects background color in Pcbnew)
Show page limits has meaning only for the schematic editor,  the board editor and the page layout editor.
Draw background color selection exists only in eeschema and  page layout editor.
In pcbnew, only a black background is allowed.
A side effect is now the user should set the background color in schematic editor, and the library editor (2 different options).
The default is still the white color.