lfg: implement CMSG_LFG_SET_BOOT_VOTE and vote-to-kick UI

CMSG_LFG_SET_BOOT_VOTE was defined in the opcode table but never sent.
- Add GameHandler::lfgSetBootVote(bool) which sends the packet
- Fix handleLfgBootProposalUpdate() to set lfgState_=Boot while the
  vote is in progress and return to InDungeon when it ends
- Add Yes/No vote buttons to the Dungeon Finder window when in Boot state

include/game/game_handler.hpp

@@ -944,6 +944,7 @@ public:
     void lfgJoin(uint32_t dungeonId, uint8_t roles);
     void lfgLeave();
     void lfgAcceptProposal(uint32_t proposalId, bool accept);
+    void lfgSetBootVote(bool vote);
     void lfgTeleport(bool toLfgDungeon = true);
     LfgState getLfgState() const { return lfgState_; }
     bool isLfgQueued()    const { return lfgState_ == LfgState::Queued; }