Feature : Current colors palette

Created dynamic palette displaying currently used colors.
This palette is now selected by default.

New service UsedColorsService created. Listens to PISKEL_RESET and
TOOL_RELEASED and keeps track of the list of current colors.

Still need to :
- improve sorting of colors
- allow to clone the current color palette as a custom palette
- add proper getter in UsedColorService to get the list of current colors

Also I hardcoded a max-height for the palettes-list as I found no better
way of handling the case of palettes containing many colors.

Maybe should add a limit to the number of colors handled by this automatic
palette.

src/piskel-script-list.js

@@ -106,6 +106,7 @@
   "js/service/keyboard/KeycodeTranslator.js",
   "js/service/keyboard/CheatsheetService.js",
   "js/service/ImageUploadService.js",
+  "js/service/UsedColorsService.js",
 
   // Tools
   "js/drawingtools/BaseTool.js",