core.Observable:fire() => add a check for string based event handlers

by default, view controllers will map those into their scope, if available.

however, in case we want to override event handlers (e.g. when defining a plugin), it can be handy to go for string based events which will get scoped to their fn once the first event fires.

example (list.Base):

afterSetStore(value, oldValue) {
    let me = this;

    value?.on({
        filter      : me.onStoreFilter,
        load        : me.onStoreLoad,
        recordChange: me.onStoreRecordChange,
        scope       : me
    });

    value?.getCount() > 0 && me.onStoreLoad();
}

could become:

afterSetStore(value, oldValue) {
    let me = this;

    value?.on({
        filter      : 'onStoreFilter',
        load        : 'onStoreLoad',
        recordChange: 'onStoreRecordChange',
        scope       : me
    });

    value?.getCount() > 0 && me.onStoreLoad();
}

in which case list.plugin.Animate can override the store event-handlers.