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.
Fix : bug when setting switching primary and secondary colors
Fix : changed classnames used for primary and secondary color highlight in palette
Renamed UsedColorsService to CurrentColorsService + exposed getCurrentColors explicitly
Moved hardcoded values to constants in PaletteListController
Merge branch 'master' into feature-dynamic-palette
Merge pull request #179 from juliandescottes/feature-dynamic-palette

Feature dynamic palette