Allow nuxeo-search-form to accept default parameters

Unfortunately, setting the params on nuxeo-search-form does not work as it does on nuxeo-results-view, which allows you to set the params attribute to a default value (e.g. params='{"param": "value"}'.

One might be tempted to manually set the params object on the search layout, on one of the polymer lifecycle callbacks (e.g. attached or ready), but this will likely fail, because nuxeo-search-form, just like nuxeo-results-view, resets the search params when the search form is loaded (or changes). However, the latter was designed to store the initial params object and use it when re-establishing the params, but the former did not. We should improve nuxeo-search-form so that we have a way to set default params.