This way, when the user changes their color theme, any sheets
that did not have a specific color set will change with the theme.
Also add a button to clear color overrides from a sheet, because
any schematics that have been saved before this change will have
the colors set to something other than UNSPECIFIED.
A note to developers, please do not set the minimum and/or size of bitmap
buttons. It causes bitmaps to get clipped using GTK on Linux. If the
bitmap button size is not acceptable, then supply an appropriately sized
bitmap rather than attempt to force the wxWidgets sizing mechanism to do
what you want. This almost always causes platform differences that do
not work well.
Includes improvements to Update PCB from Schematic dialog and
corresponding changes to Symbol Properties, Sheet Properties and
Footprint Properties.
Fixes https://gitlab.com/kicad/code/kicad/issues/4012