Adds desktop system tag to desktop systems.

[tritao]

This can be useful to use to filter desktop systems, such as os.istarget("desktop").

[tritao]

I left uwp alone as its universal, should it be tagged as both desktop and mobile?

[Jarod42]

Sad that the whole documentation is obsolete/missing for that part:

• os.istarget is missing
• os.is is deprecated and redirect to os.target...

[tritao]

Sad that the whole documentation is obsolete/missing for that part:

• os.istarget is missing
• os.is is deprecated and redirect to os.target...

Ok, I can add those missing docs and include the concept of tags too.

[tritao]

Updated the PR with some docs.

[tritao]

Updated PR with review suggestions.

Also found meanwhile https://github.com/Blizzard/premake-blizzard/blob/master/blizzard.lua#L25, so it shows some prior art as this being useful to someone else at least.

[samsinsane]

I left uwp alone as its universal, should it be tagged as both desktop and mobile?

I guess it depends on what "mobile" means? Do we factor in Windows Phone despite it being unsupported by Microsoft for years already? I don't think we should.

Does tagging with both create problems with usage? Using both filter { "system:mobile" } and filter { "system:desktop" } would create problems with systems tagged as both. While you could do filter { "system:mobile", "system:not desktop" } that becomes unreasonable if you include console which UWP could also be tagged with, or a tag for AR/VR/XR which UWP could also be tagged with.

Personally, I think uwp being tagged with desktop is fine, but others may disagree.

[tritao]

I guess it depends on what "mobile" means? Do we factor in Windows Phone despite it being unsupported by Microsoft for years already? I don't think we should.

Personally, I think uwp being tagged with desktop is fine, but others may disagree.

Ok, this sounds reasonable to me, I have updated the PR with that suggestion.

[samsinsane]

Minor issues, but otherwise looks good.

[tritao]

Thanks for the review, all should be fixed now, and updated the docs with a table reference for the tags.