Support global alt text and meta tags on assets

[vincentgercke]

Description:
There is currently no way to set alt text or meta tags globally on images/assets. If I remember correctly in DW9 you could add meta data definition to files.

Expected behavior:

• Ability to define default alt text and meta tags on an asset globally

[clauskolbak]

+1 for this - basicly making https://doc.dynamicweb.com/documentation-9/files/other/metafields or a alternative to it, work in DW 10