Added: in file dialog, multiple file selection.
    Added: Draw mode selector (in left toolbar):
        Raw mode:
            a Gerber image is drawn on screen without buffering.
            Artifacts happen if there are negative items drawn, if more than one  Gerber file is shown.
        Stacked mode:
            each Geber image is drawn in a buffer and after drawn on screen
            No artifact with negative items.
            Each Gerber image covers previous images.
        OR mode (transparency mode):
            each Geber image is drawn in a buffer and after drawn on screen
            No artifact with negative items.
            Each Gerber image is "ORed" with previous images, like in Pcbnew.
    Try to optimize Draw function in buffered modes.
        (Useful for PC that have problems with "blit" graphic function)
    Fix minor issues.