Appdata related patches

[ghost]

appdata: Use appstreamcli for appdata validation

appstream-util is obsoleted by appstreamcli.

appdata: Update appdata

• Add categories and keywords tags
• Remove duplicate Purism tag to pass validation check
• Fix duplicate custom tag error

appdata: Add more keywords

• Add several keywords that may be related the application.

[ghost]

Michael Catanzaro recommended the use of appstreamcli in this comment.

He also suggest removing the purism tags completely.

[ghost]

I'm not very familiar on Copyrights. I added them as suggestions.

    <keyword>Google Translate</keyword>
    <keyword>LibreTranslate</keyword>
    <keyword>Lingva Translate</keyword>
    <keyword>Bing Microsoft Translator</keyword>
    <keyword>Bing Translator</keyword>
    <keyword>Microsoft Translator</keyword>
    <keyword>Yandex Translate</keyword>