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/utils/UserSettings.js

@@ -9,7 +9,7 @@
     KEY_TO_DEFAULT_VALUE_MAP_ : {
       'GRID_WIDTH' : 0,
       'CANVAS_BACKGROUND' : 'lowcont-dark-canvas-background',
-      'SELECTED_PALETTE' : Constants.NO_PALETTE_ID
+      'SELECTED_PALETTE' : Constants.CURRENT_PALETTE_ID
     },
 
     /**