Add a loading message to Xpra desktop environment

[chowington]

Is your feature request related to a problem? Please describe.
After Xpra spins up and connects, it can take a while (~5-10 seconds) for the UIs of complex apps to load, during which time nothing is shown. This could lead the user to believe the app failed to launch correctly.

Describe the solution you'd like
A small window or message should immediately pop up saying that the app is loading.

[chowington]

Alternatively, we could just add a message about this behavior to the launcher page