Streamline Mac onboarding

[HebaruSan]

Motivation

Mac users have been struggling with how to start using Console UI, see #3047, #3185, and recent discussions in the macos support channel in Discord. On close inspection, not all the on-screen prompts are as clear or helpful as they could be.

Problems

If you start ConsoleUI with no instances and you don't have KSP installed in Steam, you get the KSP instance list screen:

• There's not much of a hint of what to do (the right answer is to press a to add an instance.)
• "Select a game instance" is incomplete on a screen where you can also add instances

When adding an instance:

• The fields have the default <Enter a value> ghost text, which could be more helpful
• Not all users may necessarily know what a path is, and if they do, they might not be sure about what format to use (macOS uses : as a path separator in some contexts and / in others)

Multiple users have asked for help with this exact screenshot:

• The header says "Edit KSP Instance", but we're adding
• I think they got here by dragging and dropping their KSP folder into Terminal, which "helpfully" adds backslashes on the incorrect assumption that the string will be used by bash
• I think /Applications/Kerbal Space Program is where the KSP Store installer installs KSP

Changes

• Now if you're on Mac, you don't have any instances, and your game is located in /Applications/Kerbal Space Program, we detect it automatically like we can with Steam
• New more inclusive help label on the instance list screen:
  
• Now if you don't have any instances and we can't detect the game automatically, Console UI will jump to the add instance screen instead of the empty list

When adding an instance:

• Add instance title changed from "Edit KSP Instance" to "Add KSP Instance"
• Field ghost text now gives more info about what the fields are for
  • Also done for the add repo screen, which was the only other spot where we weren't customizing the ghost text
• A help label under the path field shows <Program Files>\Kerbal Space Program or /Applications/Kerbal Space Program in your OS's path format, so you can figure out what to enter even if it's different from the default
• If you drag and drop /Applications/Kerbal\ Space\ Program onto the path field, we remove the backslashes when you try to save. Note that backslashes in general are not removed; only the ones in the specific string Kerbal\ Space\ Program because this is common and guaranteed wrong

[DasSkelett]

Yup, does a good job at pointing the users in the right directions now.