[front-end] change path to templateHTML
[front-end] put templates in separate dirs
[front-end] create templates.json, which holds list of all templates with descriptions and holders
[front-end] move templates into separate dirs
[build] copy templates.json and every template image to corresponding dir
[front-end] improve page addition controller
[front-end] add styles for page addition view
[front-end] improve page addition dialog
[front-end] dummy icons for widgets