Make notification bubble taller when body is appended

When a notification with the same summary or application name already
exists we append the text to its body and restart the timer.
As the text is appended we also increase the height with a nice
animation.

Issue: #6
Fix image_data and icon_data hints decoding

Fix image hints decoding and add an ImageItem to the QML window which is
visible when the image is not null. When we don't have an image, the
ImageItem is hidden and we show an Image with an icon from the theme
instead.

Issue: #6
Prepend file:// to image paths

Prepend the file:// scheme to image paths so that after having decoded
the hint we can tell for sure if we need to load an external file or
an icon from the theme.

Issue: #6
Wrap notification summary

Let notification summary to take two lines.

Issue: #6
Replace notification bubbles

Improve bubble replacement.

Issue: #6