Player menu: trade, group and befriend in two clicks

Added

• Right-click another player for one simple menu: Whisper, Trade, Invite to party (or Promote to leader / Remove from party when you lead, Leave party on yourself), Add friend / Remove friend, Inspect (their level and worn gear) and Ignore. It works the same everywhere: on the player in the world, on their name in chat, on a party frame, in the Party window and in your friends list.
• Greyed-out entries tell you why. If you cannot do something right now the entry stays in place, greyed out, with the reason underneath: too far to trade (come within 10 m), party full, only the leader can invite, already in a party, offline or on another world, or you are ignoring them.
• Alt + right-click opens the menu even with your weapon out. A plain right-click in combat still raises your guard, and in mouse-look right-click always guards.
• Friend notices. When someone adds you as a friend you get a ribbon with Add back or Dismiss, with a countdown, like trade and party invitations.
• The player you right-clicked stays highlighted while the menu is open. Use the arrow keys and Enter to pick an entry, and Esc to close the menu. It always opens fully on screen, even in a small window.

Changed

• Less request spam. If someone declines your trade or party request, you must wait 30 seconds before asking them again (they can still ask you). Friend notices reach the same player at most once every 10 minutes, and never reach someone who ignores you.