Gerbview: Add support for recent File Format Attribute metadata, both for X2 Gerber file format and X1 (R274X) format.
Pcbnew already uses this attribute when creating Gerber files.
Because Gerber files using this attribute identify the board layers stackup, Gerbview (if this attribute is defined) can sort gerber images stach up like the board.
(in layer manager, just right click to access to the sort menu)