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/js/Constants.js

@@ -20,6 +20,8 @@ var Constants = {
   TRANSPARENT_COLOR : 'rgba(0, 0, 0, 0)',
 
   NO_PALETTE_ID : '__no-palette',
+  CURRENT_PALETTE_ID : '__current-colors',
+  MANAGE_PALETTE_ID : '__manage-palettes',
 
   // Used for Spectrum input
   PREFERRED_COLOR_FORMAT : 'rgb',