Allow changing the menu icons

[swissspidy]

When a submenu item is moved to the top level, it automatically gets a dashicons-admin-generic icon. The user should be able to change this, ideally for every menu item, not only new ones.

Idea: A click on the icon shows a little popup next to it with a couple of icons to choose from. Selecting an icon updates the menu item and the entry in the JS array. In the PHP part we only have to detect custom icons and overwrite the existing values.

[swissspidy]

Already working, but could use a nice dropdown to select existing icons.