cheese-thumb-view use generic icon if mime icon is not available

Fix #69 (closed)

Currently, if a thumbnail cannot be created (e.g. due to a broken video file as in #18 (closed)) the generic icon for the file mime type is used.

And if this icon is not available in the current icons theme, Cheese enters in a loop trying again and again to find that icon (see #69 (closed)).

With this commit, when the icon is not available, a generic video-x-generic icon is used (even if it could happen for an image, I thought that a broken video is more likely to cause this situation. Additionally I could not find a generic icon valid for both images and videos).

And in the case the video-x-generic icon is neither available, skip this icon leaving the "waiting" icon and process the next one.

I am not familiar with coding for Gnome nor in C, so please review carefully this PR, thanks.