Xpra: Ticket #2556: rate limit xdg-menu packets

This packet sent at the start is getting close to 1MB, mostly because of all the icon data we send with each menu entry.

To fix:

add a new setting_change_attribute capability to clients
add an extra flag to the setting-change packet sent by send_setting_change so we can replace-or-update (0=replace)
send xdg-menu data one key at a time (one menu at a time), using 1=update
optional: extra marker flag for "end of updates"? (so the client knows it is a good time to update its UI)

Important for slow connections, raising.

It actually broke through the 1MB barrier today and caused the client connection to drop!

Disconnecting client /run/xpra/localhost.localdomain-2:
 packet size requested is 1719771 but maximum allowed is 1048576

Mostly because of some humongous icons.