Appdata and desktop file related changes

[yakushabb]

Please review individually.

These changes may seem significant, but they are primarily related to appdata and desktop files.

These patches accomplish three main tasks:

• Make desktop and metainfo files translatable
• Validate desktop and metainfo files
• Update metainfo file entries

In addition to these changes, this PR contains two translation-related updates:

• Addition of Turkish translation
• Update of POT file