paste_clipboard
Description:
public void paste_clipboard (Clipboard clipboard, TextIter? override_location, bool default_editable)
  
  Pastes the contents of a clipboard.
If override_location is null, the pasted text will be inserted at the cursor position, or the 
      buffer selection will be replaced if the selection is non-empty.
Note: pasting is asynchronous, that is, we’ll ask for the paste data and return, and at some point later after the main loop runs, the paste data will be inserted.
Parameters:
| this | a `GtkTextBuffer` | 
| clipboard | the `GdkClipboard` to paste from | 
| override_location | location to insert pasted text | 
| default_editable | whether the buffer is editable by default |