Provide different DPI for the icons

[tintou]

Currently the specification doesn't mention anything related to DPI support.
It seems that people are wrongly using the 128px icons for the 64@2 icons but they are not the same icons.
Here are more explicit informations about this problem:
http://blog.elementary.io/post/124193007916/whats-up-with-hidpi-icons
To solve this problem i propose to allow a dpi property that way:

<icon type="remote" width="64" height="64">http://example.com/icons/foobar.png</icon>
<icon type="remote" width="64" height="64" dpi="2">http://example.com/icons/foobar@2.png</icon>

The default dpi value should be 1 so it's not mandatory to set it.

[ximion]

We can probably have that with a scale property instead of dpi - but I don't know whether distributions would actually ship icons for 64x64, 64x64@2 and 128x128.
I could imagine for Debian to drop 128x128 and just ship the 64x64 icons and the scaled ones.
Downloading them in all sizes it quite a huge amount of data.

[ximion]

Looks like KDE Discover will have an interest in this as well, soon.

[ximion]

Please keep in mind that this is only part of the solution: The AppStream generator will need support for it too to generate another tarball with HiDPI icons, and to set the proper metadata.
With this, the initial work is done though and adding support to asgen isn't hard (and I plan to have it for the next release of it).