Set custom WM_CLASS for sandboxed apps

[dma]

trafficstars

We'll need to patch xpra to do this.

[dma]

On the Oz side of this, added support for custom environment variables as part of profile defined sandbox XServerConf: https://github.com/subgraph/oz/commit/c863b34382e494c28ee428c381352e9a67ddc699

The patch I've made to the xpra client will honor the environment variable XPRA_WMCLASS_OVERRIDE.

In an Oz profile, it would be specified like this:

, "xserver": {
	"enabled": true
	, "audio_mode": "pulseaudio"
	, "tray_icon":"/usr/share/icons/hicolor/256x256/apps/chronion.png"
	, "notifications": true
	, "env": [
		{"name":"XPRA_WMCLASS_OVERRIDE","value":"chronion Chronion"}
		]
}

[dma]

Two more TODOs here:

1. Xpra should also override class-instance server side in the off-chance that the user will run xpra client to attach manually (i.e. not through oz)
2. We should override the icon hint as well, both client and server side. If Xpra can't find icons/metadata associated by class instance, it falls back to the icon hint.