gui: Add checkboxes for alpha/opaque in surfaces view.

- pass alpha/opaque settings through to image viewer
- removed caching of thumbnails in ApiSurface instances
  - updating thumbnails (due to changed alpha/opaque settings) did not
mesh with constness
  - ApiSurfaces should be unaware of thumbnails -> caching should happen
e.g. in SurfacesView

Fixes #374.
diff --git a/gui/imageviewer.cpp b/gui/imageviewer.cpp
index 6953c00..560b477 100644
--- a/gui/imageviewer.cpp
+++ b/gui/imageviewer.cpp
@@ -10,11 +10,13 @@
 #include <QPixmap>
 #include <QScrollBar>
 
-ImageViewer::ImageViewer(QWidget *parent)
+ImageViewer::ImageViewer(QWidget *parent, bool opaque, bool alpha)
     : QDialog(parent),
       m_image(0)
 {
     setupUi(this);
+    opaqueCheckBox->setChecked(opaque);
+    alphaCheckBox->setChecked(alpha);
 
     connect(lowerSpinBox, SIGNAL(valueChanged(double)),
             SLOT(slotUpdate()));
@@ -65,9 +67,7 @@
 {
     delete m_image;
     m_image = ApiSurface::imageFromData(data);
-    m_convertedImage = ApiSurface::qimageFromRawImage(m_image);
-    m_pixelWidget->setSurface(m_convertedImage);
-    updateGeometry();
+    slotUpdate();
 }
 
 void ImageViewer::slotUpdate()