Stretch/scale icon through the properties window

[Raphael Bosshard <whistler fnord ch>]

Hello again,

this is another try with my "resize the file icon through the properties 
window". I rewrote the whole patch to fit into the general coding style 
used in fm-properties-window.h.

It will allow a user to strech/resize the file's icon through the 
properties window. This has several advantages over the "old" way, 
though it's not so "direct manipulation" anymore.

- Multiple icons can be stretched at once
- Icons can have exactly the same size

It's also more accessible than the old method.

There is one issue I was unable to fix, though: The icons will now be 
updated. I asked for help a few month ago, but some minor annoyances 
(school) distraced me from actualy fixing this issue. Help is still 
appreciated.

Thanks,
Raphael