Fix TextFields, added support for delegate, make Demo.app an Agent, general cleanup, update README

I think you know the issue with NSPopover in the Status Bar, I researched in the web about it and finally solved this problem. I also define a protocol for delegating, I think this can be useful. To simplify some tasks I also add ```togglePopover```and ```togglePopoverAnimated:```.
Demo.app is now an agent. When I make the Popover FirstResponder, the MainMenu appeared and in addition there were some troubles with fullscreen apps. This change fixes this.

In the moment there is a problem when you open an additional Popover in the Status-Popover. To resolve this, you need to close this before you close the Status-Popover. To do this, the delegate method```popupWillClose```can be used. I added a note in the README.

For better overview I added the necessary NSWindow-category to AXStatusItemPop.h und .m, so these two files are the only needed in future, too.

The other pull request isn't needed anymore. This also fixes the trouble with the Popup-Project.

I updated the Demo-Project (there are two new buttons to toggle the popover and the popover now contains a NSTextField).