Clarify the wording and explain the meaning of some labels in settings

[dragunoff]

The wording on some of the labels in settings panel is not so clear. This has come up in conversations and player feedback on Discord several times. This PR:

• Changes the label "Mute Background Music" to "Mute Menu Music" and adds a tooltip to make it clear that this is about the music in the menu when no specific track is playing. Technically this can also mute the background music in a map if it has AllowMuteBackgroundMusic set to true and hence the previous label.
• Changes the label "Player Stance Colors" to "Player Relationship Colors" and adds a tooltip explaining what the feature does. I always found this confusing as a player and had to guess what it does. Also the term stances is used for unit stances and not for player relationships (it was also changed in code - see Rename Stance to PlayerRelationship #18677 and Rename Stances to Relationships in the yaml api #18678).
• Add tooltips explaining the new transient notification options. This came up repeatedly on Discord (people couldn't figure out what the labels mean and how the changes would manifest) and was the initial motivation for this PR.

[PunkPun]

LGTM

[penev92]

Thanks! As you know I would love to see tooltips for most other settings as well 😉

[penev92]

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