Delay initializing the canPaste property until it's requested.

Delay initializing the canPaste property until it's requested.

With xcb querying data from the clipboard can take as long as 50 ms.
Deferring the check until it is requested speeds up construction time
when it's not used and only checking the available formats when it is
requested should also help.

Change-Id: I796fc83d0457d9ac1490cfb9f510c6342eb8a872
Reviewed-by: Martin Jones <martin.jones@nokia.com>