Hints for kdiff3 in main

Hints

• asv-description-first-para-too-short
  org.kde.kdiff3.appdata.xml:78 - KDiff3 is a file and folder diff and merge tool which
  The first `description/p` paragraph of this component might be too short (< 80 characters). Please consider starting with a longer paragraph to improve how the description looks like in software centers and to provide more detailed information on this component immediately in the first paragraph.
• asv-summary-has-dot-suffix
  org.kde.kdiff3.appdata.xml:60 - Unha ferramenta de comparación e fusión de ficheiros e cartafoles.
  The component summary should not end with a dot (`.`).
• asv-unknown-tag
  org.kde.kdiff3.appdata.xml:10 - developer
  Found invalid tag. Non-standard tags should be prefixed with `x-`. AppStream also provides the <custom/> tag to add arbitrary custom data to metainfo files. This tag is read by AppStream libraries and may be useful instead of defining new custom toplevel or `x-`-prefixed tags if you just want to add custom data to a metainfo file.