Added "secret" Grayscale Bitmap Builder. Shhh...

[fuglaro]

• The widget launcher button is hidden by default.
• Activates when any Editor has path set to */thumbyGrayscale.py
• Compatible with thumbyGrayscale v3.0.0.
• Outputs two bitmaps.
• The first bitmap is compatible with both thumbyGraphics and thumbyGrayscale.
• The additional second bitmap (bitmapSHD) enables grayscale in thumbyGrayscale.
• Adds js/grayscale_builder.js which is a minimally changed copy of js/bitmap_builder.js
• Differences to Bitmap Builder include: -- Zoom buttons combined together into a double button. -- 4 color picker palette added to widget space made. -- Image import support grayscale (basic without dithering). -- Import/Export behavior changed as appropriate. -- Invert behavior supports grayscale. -- Data storage unique naming to avoid data collisions with Bitmap Builder.
• A Bitmap Builder and a Grayscale Builder widget can be open simultaneously.

Thank you for considering this cheeky PR.