[js:core] add "execpre.[cmd]" event and make cmd exec cancelable

[nao-pon]

c.f. #1811

e.g. Open confirmation dialog before executing command open, tree. (Live Demo)

fm.bind('request.open request.tree', function(e) {
    var dfrd = $.Deferred(),
        cmd = e.type.substr(8),
        opts = e.data.opts;
    fm.confirm({
        title : 'Command cancel test',
        text  : 'Do you want to continue?',
        accept: {
            label    : 'btnYes',
            callback : function() {
                dfrd.resolve();
            }
        },
        cancel: {
            label    : 'btnNo',
            callback : function() {
                dfrd.reject();
                if (cmd === 'tree') {
                    setTimeout(function() {
                        // restore arrow state
                        $('#' + fm.navHash2Id(opts.target))
                            .removeClass('elfinder-subtree-loaded')
                            .addClass(fm.res('class', 'navcollapse'));
                    }, 10);
                }
            }
        }
    });
    e.data.result = dfrd;
});