First version of configurable pastebin applet

Now we can create multiple backends for the different "pastebin servers" out there. When creating a new backend, one just needs to add it to backend.h and implement server.h API. The backend must emit a signal postFinished(const QString&) with the URL to be displayed in the applet.

Commit also implements another backend (pastebin.com). More to come.

Now we just put the URL in the clipboard, instead of the whole text that is displayed.