custom icon for mimeIcon

First I want to thank you for this tool, it's very powerful and extremely useful to me ❤️
**Is your feature request related to a problem? Please describe.**
I would like to implement a custom command that allows me to search for images to paste. After searching, I was planning to use the menuItems function to prompt the user to select which image they would like to paste on the clipboard. It is my understand that menu items can specify a custom icon, but it must be from a predefined list, [ie in here](https://github.com/hluk/CopyQ/blob/fd3fb8e1ef3876ed6a5b15f47bbe740e01543d4d/src/gui/icon_list.h#L1563).

**Describe the solution you'd like**
If there's not a better way to approach this, it would be nice to be able to specify a custom icon eg encoded in base64

**Describe alternatives you've considered**
dialog() with custom icon "Icon for custom dialog can be set from icon font, file path or theme" for each search result with a yes/no - this would be a bit clumsy