Fix tray icon menu position and color on Linux (Gnome)

[HebaruSan]

Problem

#2565 added a tray icon with a context menu. On Linux, it has light text on a white background and it can be displayed mostly off-screen:

(The icon is invisible as well. I don't know why that is, so this pull request doesn't attempt to fix it.)

Cause

Mono?

Changes

Now the background matches other menus, and the position is always on screen:

In addition, the tray icon related code is moved from Main.cs to MainTrayIcon.cs.

[politas]

Which tray does it go in?

[HebaruSan]

Depends on your desktop, I don't know that much about MATE. Maybe in one of the task bars, near the clock?

[politas]

I've got an Indicator Applet and an IBus applet, that show different "tray" stuff.

[HebaruSan]

I guess I'd expect it in the indicator applet? Mine for GNOME Shell is in the TopIcons Plus extension. But if you see a suspicious blank space anywhere, try right clicking it.

[politas]

Nope, doesn't show up anywhere, and then refuses to unminimise
Correction: My "Notification Area" applet expands a little, but the space shows an echo of the neighbouring icon and doesn't respond to any mouse clicks.

[politas]

I'll just have to take your word that it works on Gnome.

[politas]

I may well be the only Mate user of CKAN, and I don't care about this feature, as it doesn't have the main element I wanted for a tray applet (handling multiple KSP installs).