Remove premature (and bad) optimization

When blinking the caret, it looks like a cool idea to only update the
small rectangle containing the caret. Actually it is an awful idea, since
the paint event will paint some rows, and these rows will not be
properly painted outside of the small rectangle. Unfortunately, the
painter will skip those "painted" rows on next paint events.

This leads to painting errors that depend on the ordering of "real"
and "caret" paint events. This is the reason why they only appeared
with split screens.

Quote of the day: ``The real problem is that programmers have spent far
too much time worrying about efficiency in the wrong places and at the
wrong times; premature optimization is the root of all evil (or at
least most of it) in programming.''
-- Donald Knuth, The Art of Computer Programming.

Fixes bug #11354.

src/frontends/qt4/GuiWorkArea.cpp

@@ -639,7 +639,7 @@ void GuiWorkArea::Private::showCaret()
 		return;
 
 	updateCaretGeometry();
-	p->viewport()->update(caret_->rect());
+	p->viewport()->update();
 }
 
 
@@ -650,7 +650,7 @@ void GuiWorkArea::Private::hideCaret()
 
 	caret_visible_ = false;
 	//if (!qApp->focusWidget())
-		p->viewport()->update(caret_->rect());
+		p->viewport()->update();
 }