Rewrite templates for different widgets to make them composable

As of now different fields templates do not use composition which increases template complexity and limits widgets functionality. A simple-example: there is a template for dictionary widget https://github.com/tsufiev/merlin/blob/master/merlin/static/merlin/templates/fields/dictionary.html and for string widget https://github.com/tsufiev/merlin/blob/master/merlin/static/merlin/templates/fields/string.html

To enable autocompletion functionality for separate fields of dictionary (given that it's already supported on the model-level) it's necessary to modify the dictionary template, while in case the dictionary template was composed of other <typed-field> directives it could delegate rendering autocompletion hints to the string widget (that's the argument for 'limited functionality').

A strong argument for 'increased complexity' is the contents of https://github.com/tsufiev/merlin/blob/master/extensions/mistral/static/mistral/templates/fields/varlist.html which repeats most parts of string, list and dictionary templates depending on the value of this wigdet's type.

Most likely this refactoring should start from the top-level, leaving there only iteration over panels and hiding all other 'ng-repeat's into <typed-field type='panel'> innards