Use a better default for the run dialog

[allo-]

Describe the bug
The "server -> run command" dialog seems to use the first item in the menu, when used the first time after connecting to the server. In my case this is "Alternatives Chooser", a program that I rarely need. The box could be empty or have a more useful default like the last command used.

To Reproduce
Steps to reproduce the behavior:

1. Connect to the server
2. Click the tray icon
3. Select Server -> Run command
4. The dialog has an input box prefilled with an item from the menu, probably the first in alphabetical order

System Information (please complete the following information):

• Server OS: Debian stable
• Client OS: Debian testing
• Xpra Server Version 6.2
• Xpra Client Version 6.3