Enhance Palette panel/editor

[novhack]

Currently editing of a palette is done in a dialog. Most of other software allows users to add and change colors directly from the main UI which is in my opinion a better workflow. I propose these:

Changes:

• [x] Plus/Minus buttons to add/delete colors in a Palettes panel. Left click on Plus button adds left tool color, right click adds right click color to the end of the palette.
• [x] Double click on a color opens color editor dialog.
• [x] Remove delete palette button from panel and move it to edit palette dialog. It's a feature that is most likely not used a lot.
• [x] Move and copy colors directly in the panel.
• [x] Drag and drop on existing color = swap colors.
• [x] Drag + shift = insert color in between two existing colors.

Additions:

• [x] Ability to Change palette grid size.
• [x] Organize colors freely on the palette grid (with empty spaces).
• [x] Drag + ctrl = copy color to empty grid space.

Future improvements:

• [x] Replace current JSON palettes implementation with TRES.
• [ ] Add palette modifications to undo/redo system.
• [ ] Import palettes based on file headers not file extension. Files like palette.gpl.txt should be imported correctly.
• [ ] Indexed colors mode. Changing a color in the palette changes the same color in the canvas. #171
• [ ] Colors multi select
• Ability to select multiple colors and use scroll wheel to iterate through them.
• [ ] Colors sorting.
• [ ] Color gradients generation.
• [ ] Select multiple colors and generate one color that has their average value. This workflow can help with cleaning up images. Hue Shifting in Pixel Art (Color Tutorial)
• [ ] Support for project palettes. Embedded into .pxo file for better portability.
• [ ] Merge palettes
• A dialog with check boxes that allows user to merge all colors from other palettes to a current palette

[novhack]

Issue #187 was already suggesting some of the changes I described.