#605

The theme file is now project specific because Angular demands assets
to be part of the project.
It also differs in content: The editor theme is only applied to canvas
elements. For the player every checkbox and radio button is affected.

The components now again use the default color of 'accent'. The custom
theme now has the green as accent color and no longer primary.

* Delete validation message component
* Use instead `mat-error` inside element components to take advantage
 of `mat-input-field`
* Add get validators method to each form element component
* Display validation messages via `error-transform` pipe

Move `MatDialogModule` from editor to common
Add `AlertDialog` to player

Refactor some translations