Expose clicks on the systray icon.

[tommi-hack]

I want to use systray icon without menu. Instead, I would like to get some notification about the click on systray icon.
Especially good would be if click returned x, y coordinates of the potential menu.

Featre request:

1. Make it possible to pass in channel that would receive event on systray icon clicks.
2. make the channel to return menu location

Theoretically it could be something like this (in meta code):
iconClicks := systray.GetSystrayClicks() // it will return: chan struct{x: int, y: int}

Specifically I am building app with "fyne.io/fyne/" and want fyne app window open after clicking on the systray icon. And the window should be aligned to the systray icon. Similar to Dropbox app.

There are several similar requests made but with a bit different details:

#175
#121

And one PR for Windows only:
#160