New party mode: Partner Swap (#915)

* Add partner swap party mode

A new party mode for 3 to 12 players that only needs 2 microphones. Everyone plays for themselves, but always sings together with a partner. Both singers get the score of the pair, and the partners change every round.

- In each round every player sings once, with a partner they haven't sung with yet; with an odd number of players someone takes a break, in turns
- Ranking by the average score per song, so breaks are no disadvantage
- Optional harmony bonus of 500 points when both singers are no more than 500 points apart
- Duet songs are sung with both voices; duets can be mixed in, used exclusively or left out
- Up to 3 jokers per player to skip a song
- The best pair is shown all the time, and the winner and the best pair of the night are celebrated at the end

* Rename the harmony bonus points constant to avoid shadowing

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Co-authored-by: Florian Ostertag <flokuep@users.noreply.github.com>

Output/PartyModes/PartnerSwap.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="utf-8"?>
+<root>
+  <PartyModeSystemVersion>2</PartyModeSystemVersion>
+  <Info>
+    <Name>TR_PARTYMODE_NAME</Name>
+    <Author>McSchneidar</Author>
+    <Description>TR_PARTYMODE_DESCRIPTION</Description>
+    <Folder>PartnerSwap</Folder>
+    <PartyModeFile>CPartyModePartnerSwap</PartyModeFile>
+    <PartyModeVersionMajor>1</PartyModeVersionMajor>
+    <PartyModeVersionMinor>0</PartyModeVersionMinor>
+    <TargetAudience>TR_PARTYMODE_AUDIENCE</TargetAudience>
+  </Info>
+  <PartyScreens>
+    <ScreenFile>CPartyScreenPartnerSwapMain</ScreenFile>
+    <ScreenFile>CPartyScreenPartnerSwapConfig</ScreenFile>
+    <ScreenFile>CPartyScreenPartnerSwapNames</ScreenFile>
+    <ScreenFile>CPartyScreenPartnerSwapSongs</ScreenFile>
+  </PartyScreens>
+</root>