fix(core): fixed routerAjaxStart is never triggered

[xxxCam900xxx]

Issue: routerAjaxStart never triggered #4303

This commit should provide you with a trigger for the 'routerAjaxStart' event, which you can also use to change its options.

Example:

var app = new Framework7({
	on: {
		routerAjaxStart: function (options) {
			options.headers = new Headers();
			options.headers.set('custom-header', 'custom-header-value');
		},
	}
}