Rename Painter::set_clip_rect() to add_clip_rect().

It was confusing to see multiple calls to set_foo() in a row. Since this is
an intersecting operation, let's call it add_clip_rect() instead.

Applications/ProcessManager/MemoryStatsWidget.cpp

@@ -105,6 +105,6 @@ void MemoryStatsWidget::timer_event(GTimerEvent&)
 void MemoryStatsWidget::paint_event(GPaintEvent& event)
 {
     GPainter painter(*this);
-    painter.set_clip_rect(event.rect());
+    painter.add_clip_rect(event.rect());
     StylePainter::the().paint_surface(painter, rect());
 }