Enhancement : Shortcuts for onion kin + layer prev

Added 2 new icons :
- Animated Preview Panel : toggle onion skin
- Layers Toolbox : toggle layer preview

Added an icon font generated with icomoon.

SVGs for both icons have been made under Inkscape and are kept under
misc/svg.

All reference SVGs have been moved to misc/svg.

Added 2 keyboard shortcuts for toggling onion skin / layer preview :
- alt L : toggle layer preview
- alt O : toggle onion skin

src/piskel-style-list.js

@@ -3,6 +3,7 @@
 (typeof exports != "undefined" ? exports : pskl_exports).styles = [
   "css/reset.css",
   "css/style.css",
+  "css/font-icon.css",
   "css/forms.css",
   "css/settings.css",
   "css/tools.css",
@@ -13,6 +14,7 @@
   "css/toolbox.css",
   "css/toolbox-layers-list.css",
   "css/toolbox-palettes-list.css",
+  "css/toolbox-animated-preview.css",
   "css/spectrum/spectrum.css",
   "css/spectrum/spectrum-overrides.css",
   "css/bootstrap/bootstrap.css",