Add appdata file

[AsavarTzeth]

This is used by Software Centers and packaging systems that make use of
the AppStream specification. It adds distribution independent metadata
for the project.

Screenshots are not supplied due to the extra work needed to make new
ones that comply with the AppStream recommendations. Specifically the
difficulty lies in only showing the window, yet maintaining an aspect
ratio of 16:9.

Not sure how this is best solved. Likely the only solution is to
something similar to the current screenshot.jpg, minus the background
and no padding.

This appdata file validates using appstream-util validate-relax.

Should fix #121

[AsavarTzeth]

The description might also need some improvements. Currently it is based on the description on the original homepage and the README. But at least now you have the fundamentals.

[JMoerman]

Thanks for the pr! I am aware that the current screenshot on this repo, I'll make one without background soon.

[AsavarTzeth]

Some more tips for the screenshots:

• Official recommendations: https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html#qsr-app-screenshots-info
• Make use of win_width= and win_height= in go-for-it.conf. Since it is recommended to "not scale screenshots below their original size", combined with the 16:9 aspect ratio, being able to set this in the app before the capture is quite useful.

[JMoerman]

Sorry for taking this long to respond, I've updated the screenshot to

I had already read those recommendations, but I didn't follow all of them as I did not see a way to use an aspect ratio of 16:9, show a single aspect of the application and have it look good at the same time.
The aspect ratio is 16:9, but I show 3 different views of the application.

[AsavarTzeth]

Very nice. A minor thing worth noting is that appstream-util validate, part of appstream-glib, does not like vertical padding:

• style-invalid         : <image> has vertical padding [https://raw.githubusercontent.com/JMoerman/Go-For-It/master/screenshot.png]

However this only affects that implementation of the specification. Meanwhile appstreamcli validate, part of AppStream has no issues. I think warns about this because Gnome Software might add some padding itself.

Anyhow, under the strictest validation tests, that is now the only warning. There is no requirement to pass this and since it would in this case be difficult to otherwise get a 16:9 aspect ratio, it can probably be ignored.

[AsavarTzeth]

Oh there was actually one other warning that you managed to add.

    The component summary should not end with a "." [A stylish to-do list with 
    built-in productivity timer.]

[JMoerman]

Oh there was actually one other warning that you managed to add.

    The component summary should not end with a "." [A stylish to-do list with 
    built-in productivity timer.]

That is now fixed: f02cb79