feat(TileEditorView): add tile picker for paint tool
feat(TileEditorView): add layer select and visibilty toggle actions

- Support selecting active layer with a click/touch and toggling visibility by clicking on the eye icon.

TODO: options on which actions to track as undoable?
refactor(build): remove pow2 and add pow-core as a bower dependency

- Adjust XML loader and other pow-core usages accordingly
- Remove Backbone.js references (pow-core implements an events class)
- Underscore types are include with pow-core.