Support image-path hint

[bebehei]

As GApplications expose their icon via image-path hint and not via the
actual app_icon field, GApplication icons haven't been displayed before.

According to the notification spec, image-path hint should also override
the app_icon as follows:

1. image-data hint
2. image-path hint
3. app_icon parameter

[bebehei]

Fixes #446

[tsipinakis]

The priorities are a bit backwards here. In x.c the raw icon always takes priority over the path but that's not how it is in the specification:

• image-data (raw icon)
• image-path (icon)
• app_icon (icon)
• icon_data (raw icon)

[tsipinakis]

I've just realized that this also affects the current master and is thus out of scope for this, I'll approve this but we should handle this point too.

[bebehei]

The priorities are a bit backwards here. In x.c the raw icon always takes priority over the path but that's not how it is in the specification

Except for icon_data, we handle everything right. IMHO it's ok to "ignore" the spec here and care about more important things.