Add SPDX header

[AndyScherzinger]

and...

• a GH action to check for future compliance on PRs
• replaced icons with latest material ones

Note that SVGs aren't optimized (can't tell if needed or how to do it.)

Also looping in @jancborchardt if the icon changes especially the app icon, are fine.

Changed icons are:

• https://github.com/nextcloud/photos/pull/2459/files#diff-ab82ac108cd0ae5aef1cc0fb3b35ec3984561af212f4e2b5b3636185160d6b28
• https://github.com/nextcloud/photos/pull/2459/files#diff-dbfad3874b2d221216a6d83c959358e200d07fafaf39c45522c9fe67efae5ef1
• https://github.com/nextcloud/photos/pull/2459/files#diff-28199908ac986fe6b542c6a07927aa7eea3edb635a99a3a18528948bab33ea6b
• https://github.com/nextcloud/photos/pull/2459/files#diff-48e4b53d454342d52be3e79a9982086dad2b0913bb74e9e7cafa8942a8f8edd2
• https://github.com/nextcloud/photos/pull/2459/files#diff-6f11f5efbb9f966b324f3a313de9ad45728a15c0b00a3f81eecf6acb199833e5

[AndyScherzinger]

/compile /

[artonge]

/compile /

[jancborchardt]

In this case it’s the outline versions of the Material Design icons, but they actually look better here. So all good.