make it easier to disable almost everything

[totaam]

Related to #3835 and #3952, and also useful for limited builds like #3945, debugging, memory savings, etc..

This should give the user a working session, capable of doing the basics (window / desktop forwarding) but without all the bells and whistles, disabling:

• audio and video
• notifications
• tray forwarding
• mmap
• webcam
• file-transfers
• rfb / ws / ssl / ssh socket upgrades
• mdns
• opengl
• shell
• dbus
• remote logging
• starting new commands
  etc

Perhaps even:

• clipboard
• control commands
• most encodings? (keeping just pillow?)

[totaam]

Done in the commit above.
Usage examples:

xpra start --minimal ...

xpra attach --minimal