Update previews after pasting (#6211)

The reason we want to do this is for if there is pasting across buffers.
For within buffer, the previews should already be updated. Calling the
buffer-wide updatePreviews() might seem like overkill, but actually it
should be quick because only the previews that need updating are updated
(and this check feels quick to me, although I did not profile).

If we were to loop through the pasted text and update each preview
individually, this might take more time. When updating previews
together, only one .tex file is compiled.

src/Text3.cpp

@@ -1312,6 +1312,7 @@ void Text::dispatch(Cursor & cur, FuncRequest & cmd)
 		bv->buffer().errors("Paste");
 		cur.clearSelection(); // bug 393
 		cur.finishUndo();
+		bv->buffer().updatePreviews();
 		break;
 	}