Add a control command to set environment variable used to start new processes

[normanr]

Is your feature request related to a problem? Please describe.
I'm always frustrated when I forget to set an environment variable before starting the xpra server, because all programs that I start run without the environment variable until I restart the xpra server (a simple examples is changing SSH_AUTH_SOCK after restarting ssh-agent).

Describe the solution you'd like
I'd like to be able to set the environment variables used for starting new applications via xpra control after I've started xpra.

Describe alternatives you've considered
I can just restart xpra, it's a pain. gnu screen allows me to run :setenv, so why shouldn't xpra allow something similar.

[totaam]

Was already on my TODO list. Even better to have a ticket for it.

[totaam]

@normanr Done, here are some examples:

xpra info :10 | grep start-env
xpra control :10 start-env unset GTK_CSD
xpra control :10 start-env set GTK_CSD 1
xpra info :10 | grep start-env.GTK_CSD

[normanr]

👍

I found a workaround, which was to export the variables I want and then run xpra upgrade. It works well enough, even though it's kinda ugly (and causes your xpra connection to drop), at least it doesn't restart the X server.

[totaam]

@normanr when you xpra upgrade, the client should notice and automatically re-connect.

[normanr]

Manually reconnecting isn't a big issue. Also I have automatic reconnect disabled, because establishing the connection requires me to be present to confirm use of my ssh key.