Directly offer a QIcon from the component

[aleixpol]

Instead of giving the different pieces (which could also be accessible
through the API)

[ximion]

This introduces a dependency on GUI stuff which I was very careful to avoid so far...
I guess the Qt library is highly likely used with GUI stuff in almost all the cases though, unlike the C library which is also used without any GUI stuff at all, which might make this a useful thing for the Qt lib...

Issues I see here are though:

• No support for remote icons, and introducing it will be trickier
• No way to select a specific icon size
• We limit ourselves to the things a QIcon can do, for better or worse.

I need to sleep and think about this tomorrow when I'm more awake.

[ximion]

I added a non-GUI icon implementation which also works with remote icons (which will be used much more in future, I assume).
We can still think about adding something QIcon based additionally though, if a GUI dependency is okay for this library...