metainfo improvements needed

[okias]

$ appstream-util validate-strict srain.appdata.xml   
srain.appdata.xml: FAILED:
• tag-missing           : <translation> not specified
• tag-missing           : <update_contact> is not present
• style-invalid         : <image> has vertical padding [https://srain.im/assets/images/cover.png]
• style-invalid         : <image> has horizontal padding [https://srain.im/assets/images/cover.png]
• aspect-ratio-invalid  : <screenshot> aspect ratio not 16:9 [https://srain.im/assets/images/cover.png]
• style-invalid         : <caption> is too short [Main UI];shortest allowed is 10 chars
• style-invalid         : <image> has vertical padding [https://doc.srain.im/en/latest/_images/srain-startup.png]
• style-invalid         : <image> has horizontal padding [https://doc.srain.im/en/latest/_images/srain-startup.png]
• aspect-ratio-invalid  : <screenshot> aspect ratio not 16:9 [https://doc.srain.im/en/latest/_images/srain-startup.png]
• style-invalid         : <caption> is too short [Login UI];shortest allowed is 10 chars
• style-invalid         : <image> has vertical padding [https://doc.srain.im/en/latest/_images/srain-search-channel.png]
• style-invalid         : <image> has horizontal padding [https://doc.srain.im/en/latest/_images/srain-search-channel.png]
• aspect-ratio-invalid  : <screenshot> aspect ratio not 16:9 [https://doc.srain.im/en/latest/_images/srain-search-channel.png]
• style-invalid         : <summary> cannot end in '.' [Modern IRC client written in GTK.]
• translations-required  : <name> has no translations
• translations-required  : <summary> has no translations
• translations-required  : <description> has no translations
Validation of files failed

[SilverRainZ]

:'( These rules are quite strict.

[SilverRainZ]

#244 fixes some issues of them, but not all.

I am not sure how to fix a "translations-required" error.
Some "style-invalid" errors about image are quite old fashioned IMO so I won't fix them.