Multiselect functionalty targeting https://github.com/owncloud/android/issues/596

Multiselect functionality only works on api level 18+ and on applications that has included that functionality. As pointed out in the issue it may not be clear for the user why it works in some applications and not in others, but until you have decided how to handle that this code adds multiselect for apps that support it. (multiselect has been tested on Google+ Photos app)