Replace mailbox menu icon classes with material icons

[ChristophWurst]

Ref nextcloud/groupware#38

Before

After

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Like the bikeshedding about which icon for what action begin.

[GretaD]

tested looks good

[jancborchardt]

Like the bikeshedding about which icon for what action begin.

Yup @ChristophWurst @GretaD, better icon suggestions:

• Folder info: The icon of the folder itself instead of the generic info icon
• Mark all as read: https://fonts.google.com/icons?selected=Material%20Icons%3Adone_all%3A
• Add submailbox → Add subfolder: https://fonts.google.com/icons?selected=Material%20Icons%3Afolder%3A – plain folder icon is enough. (And there are 2 different icons in the 2 "after" screenshots?)
• Edit name → Rename: https://fonts.google.com/icons?selected=Material%20Icons%3Aedit%3A

Other ones are good 👍