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/js/utils/UserSettings.js

@@ -30,8 +30,12 @@
     get : function (key) {
       this.checkKeyValidity_(key);
       if (!(key in this.cache_)) {
-        this.cache_[key] =
-          this.readFromLocalStorage_(key) || this.readFromDefaults_(key);
+        var storedValue = this.readFromLocalStorage_(key);
+        if (typeof storedValue !== 'undefined' && storedValue !== null) {
+          this.cache_[key] = storedValue;
+        } else {
+          this.cache_[key] = this.readFromDefaults_(key);
+        }
       }
       return this.cache_[key];
     },