ios feature for getting / setting the clipboard string

- ofxiOSSetClipboardString() to set clipboard string.
- ofxiOSGetClipboardString() to get clipboard string.
- including ofxiOSExtras.h in ofAppiOSWindow so @bakercp can add these
methods to the general clipboard methods extended from ofAppBaseWindow
Merge pull request #2451 from julapy/ios-clipboard-string

- added ofxiOSSetClipboardString() to set clipboard string.
- added ofxiOSGetClipboardString() to get clipboard string.
- including ofxiOSExtras.h in ofAppiOSWindow so @bakercp can add these
methods to the general clipboard methods extended from ofAppBaseWindow