NPC base class: migrate all branch-state methods

[jackmakesthings]

Closed the main migration tasks already; opening this one to double-check that all the needed functionality (_at_state, update_and_continue, that family of features) are set up correctly. Functions from the old prototype:

NPC

• [x] setup_MUI and set_MUI_portrait move to MUI?
• [x] decline_conversation (aka i_should_go) end_interaction
• [x] present_conversations
• [x] init_branch
• [x] set_attractor deprecated; new method of 'attracting' doesn't mess with the tilemap
• [x] stop_attracting see previous
• [x] redirect_player rolled into _on_click
• [x] _on_click
• [x] _stopped_nearby moved into Utils as is_player_nearby
• [x] start_interaction
• [x] has_conversation(branch) check_branches/Branch.has_active_state
• [x] set_branches now handled by managers and quests themselves, not by NPCs
• [x] check_branches
• [x] set_paths removed; moving away from this style of setting node refs
• [x] npc_ready removed until it proves its usefulness again
• [x] _ready and _enter_tree updated and/or redundant