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:

• 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.
• Double click on a color opens color editor dialog.
• 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.
• Move and copy colors directly in the panel.
• Drag and drop on existing color = swap colors.
• Drag + Shift = insert color in between two existing colors.

Additions:

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

Future improvements:
Replace current JSON palettes implementation with TRES EDIT: Reverted in #967.

• Palette grid zoom and resizing
• Colors sorting.
• Colors multi select
• 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)
• Indexed colors mode. Changing a color in the palette changes the same color in the canvas. Feature: Index color mode, (pallette changes can impact colors already painted) #171
• 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.
• Ability to select multiple colors and use scroll wheel to iterate through them.
• 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]

#761 implements grid zoom and resizing features