Dalamud Link Handler

[KazWolfe]

Adds preliminary support for the Dalamud Link Handler (using the concept at goatcorp/Dalamud#2462, and protocol from goatcorp/Dalamud#2468). Only a start, but it at least is an okay one, I hope.

• New start param for TempDirectory that allows XL to hint to Dalamud where to put its temp files
  • Defaults to $TEMP/XIVLauncher.
• New project XIVLauncher.LinkHandler for handling inbound links.
  • Command line option -d to specify a custom search path, otherwise uses above default temp path.
• Add RPC to XIVLauncher.Common for ease of use elsewhere.

Future Art

• Add a job to auto-delete dead sockets if they aren't cleaned up by the game.
• UX pass for the link handler
• Add link handler to HKCU's protocol handler registry.
• Make building less terrible.

[KazWolfe]

goatcorp/Dalamud#2526. Support dropped.