Enhancement : Rename layers

- added rename icon in layers list
- mutualized CSS for edit icon between palette manager and layers list
- new CSS file icons.css for actions/links/buttons using icons
- layers are no longer retrieved by name, but by index

src/piskel-style-list.js

@@ -6,6 +6,7 @@ exports.styles = [
   "css/forms.css",
   "css/settings.css",
   "css/tools.css",
+  "css/icons.css",
   "css/cheatsheet.css",
   "css/dialogs.css",
   "css/dialogs-manage-palettes.css",