Use "start-here-mate" for the menu icon for better theme compatibility

[gabdub]

I noticed that the icon of the menu can no longer be changed and is now set to the icon provided by the theme as "start-here".

The change was asked here as an option: #45

The problem I have is that I want to use "Numix circle" as my theme, but I don't want to have a red circle with an "n" in the panel (the left part of the image is my current setup and the right one is UM19.10 running in a VM).

Numix is a theme that can be applied to many distributions. They provide a generic "start-here", a version for Arch ("start-here-arch.."), a version for KDE ("start-here-kde" and a symbolic version "start-here-symbolic".

As @vkareh suggested here:
https://ubuntu-mate.community/t/ubuntu-mate-19-10-beta-testing/20262/129

GTK allows you to load icon fallbacks by removing hyphen parts: start-here-mate -> start-here -> start -> missing-image, so this should satisfy pretty much all themes.

[vkareh]

Thanks for reporting this. To be specific, the fix is to submit a start-here-mate icon to the numix project so that we can use that. The first step is to create such an icon 😄

[gabdub]

Numix has an icon called "desktop-environment-mate" with the mate logo already. I think they can link it from "start-here-mate".

[vkareh]

Here are the ones I found:

• https://github.com/numixproject/numix-icon-theme-circle/blob/master/Numix-Circle/48/apps/distributor-logo-ubuntu-mate.svg
• https://github.com/numixproject/numix-icon-theme-square/blob/master/Numix-Square/48/apps/distributor-logo-ubuntu-mate.svg
• https://github.com/numixproject/numix-icon-theme-circle/blob/master/Numix-Circle/48/apps/desktop-environment-mate.svg
• https://github.com/numixproject/numix-icon-theme-square/blob/master/Numix-Square/48/apps/desktop-environment-mate.svg

@flexiondotorg - I know there was a reason to not use the distributor-logo icons, do you remember what that was? I think start-here is more contextually appropriate, but want to figure out what's most compatible with other well-known themes.