Appdata related patches

[yakushabb]

appdata: Add branding colors

appdata: Update screenshot

• Fix screenshot URL
• Add a caption

appdata: translate=no properties

It appears that the appstream project no longer supports
translatable=no properties, and gettext extract the
translatable=no marked strings as translatable.

I opened an issue to inform about the situation, but translatable=no
properties are not accepted by developers. You can find the issue
here: https://github.com/ximion/appstream/issues/623

Please test your script or string extraction process before merging this PR.

In MetaInfo files, each individual paragraph of a description
(or enumerated entry) is translated individually, however,
you can only exclude the complete block from being translated
by adding translate="no" to the description element.

Source: https://freedesktop.org/software/appstream/docs/sect-Quickstart-Translation.html

[yakushabb]

Also Fixes: #9

[SeaDve]

Thanks, I appreciate the helps, as always! But, like the other apps, ninja -C _build kooha-pot seems to ignore it. You could create a separate PR, so I could merge the other commits.

[SeaDve]

Hi, thanks for the PR. However, may I request a rebase, thanks!

[SeaDve]

Thanks!