Handles CTRL-Enter event to save in the editor

Many webapplications (Github for example) use CTRL-Enter as a short cut to submit the current form. This pull request adds that functionality to DokuWiki: Press CTRL-Enter in the editor and editor.js clicks for you on the #edbtn__save-button.

It should be compatible with all themes which don't change to id of the save button. And shouldn't interfere with those that did change the id.

Tested on Mac OS X Yosemite 10.10 with Chrome 38 and Safari 8 and on Windows 7 with Internet Explorer 11, Chrome 38 and Firefox 33.