[Request] Image / Texture / Pattern Swatch Panel

[Jose-Moreno]

--Issue Summary--

Similar to the color palette, I feel we could also have a texture / Pattern / image loader palette where you can load different images for quick access to drag & drop them directly on Pencil2D canvas.

This would work particularly well for any image that is part of a project file for an animation. Things like:

1. Color Models (Character Color-Key Concepts)
2. Field Charts
3. Layout Guides (which vary for different studios)
4. Background Paintings
5. Storyboard Panel Templates
6. Paper /Canvas Textures to be used for the texture picker proposal in the preferences (see #990 )
7. And more! ...

I feel the implementation could be taken form the color palette panel and the color swatches replaced by thumbnails, but the functionality would be the same, except for allowing themselves to be dragged & dropped on the canvas.

Will update this proposal later with more fleshed out ideas.

[scribblemaniac]

Partially related to #571