mainwindow: scale icons to sane size

If load_icon() fails, we treat the icon name as a file path and try to load an image from the path. In case that works, we need to ensure that the has correct size. Previously that wasn't done, which led to too large icons.

scale_simple() doesn't do anything if the image is already the correct size, so we can call it unconditionally.

The exception handling was a bit weird in that the exception types didn't match the documentation of IconTheme::load_icon() and Image::set(). I updated the exception types (Image::set() doesn't need exception handling any more, because now it's called with a Pixbuf rather than a file name).

Fixes: #60 (closed)