Add some steps to verify the appdata metadata

[Abestanis]

This depends on #580, because with the current screenshot the verification fails with the following error:

src/res/com.ulduzsoft.Birdtray.appdata.xml: FAILED:
• attribute-invalid     : <screenshot> width too large [https://raw.githubusercontent.com/gyunaev/birdtray/master/screenshots/birdtray-settings.png] maximum is 1600px
• attribute-invalid     : <screenshot> height too large [https://raw.githubusercontent.com/gyunaev/birdtray/master/screenshots/birdtray-settings.png] maximum is 900px
• style-invalid         : <image> has vertical padding [https://raw.githubusercontent.com/gyunaev/birdtray/master/screenshots/birdtray-settings.png]
• style-invalid         : <image> has horizontal padding [https://raw.githubusercontent.com/gyunaev/birdtray/master/screenshots/birdtray-settings.png]
Validation of files failed

The errors disappear with the new screenshot, see the CI on commit 540baee.

[Abestanis]

This is ready to be reviewed, but I'll leave it marked as a draft until #580 is merged.