Alternate game command lines and Steam refactor

[HebaruSan]

Background

If you're a Steam user, you can add extra game instances to Steam using this menu item:

Motivations

• @linuxgurugamer wants to be able to configure an alternate command line that runs the game with a debugger
• @k0smonaut was surprised that launching the game through CKAN didn't make Steam count play time. This is because simply launching KSP_x64.exe or KSP2_x64.exe doesn't launch it through Steam. To count play time in Steam, you have to launch the game with Steam, which can be done either by clicking in Steam or with a steam://rungameid/ URL.

Changes

• Now all of our disparate Steam handling code is refactored into a single SteamLibrary class that uses the newly referenced ValveKeyValue MIT-licensed library to retrieve Steam's full list of games from its data files
• Now on the first run, instead of scanning each of your configured Steam library folders for one hard coded directory name per game, we check for each supported game in each Steam game folder, including non-Steam games, so if you have multiple game instances added as non-Steam games, they'll all be found
  • The instance names will be pulled from the names Steam uses for each game instead of "Auto KSP" or "Auto KSP2"
  • We now retrieve the library folders from config/libraryfolders.vdf instead of config/config.vdf because @Olympic1's additional folder was listed in the former but not the latter
  • Since this scan checks that the game is actually installed, it will make us somewhat more robust in the face of unusual situations like SteamApps/common/Kerbal Space Program existing but being empty, which would have caused problems under the old logic but would correctly be ignored now
  • There is also a new "Import from Steam" option in the "Add new game instance" dropdown that will perform the same scan and add any instances it finds that aren't already in your list, to make it easy to "catch up" in case users add non-Steam games to Steam after running CKAN
    
• Now IGame.DefaultCommandLine is replaced by IGame.DefaultCommandLines, which uses the SteamLibrary class to find the URL for each game when available in addition to the direct EXE file option
• Now the command line setting is a list instead of a single value. When you first run a version of the CKAN client with these changes, your currently configured command line will be pulled forward into this setting, with the default command lines pulled in afterwards. A new "Reset to defaults" button is available in case you want to revert to the default command line settings.
  
• Now the Play button has a dropdown menu showing all of your configured command lines and some help text indicating whether each one will count play time in Steam if you have Steam, followed by an edit option to make it easier to find the command line settings
  

Side fixes:

• ckan update won't spam "Loading modules from downloaded default repository..." anymore
• The GUI mod list will no longer preserve which row was selected when switching from one instance to another, because it's distracting to jump halfway down the list and not know why

Fixes #2706.
Fixes #4003 (in that it explicitly tells the user whether Steam will count play time and makes the steam:// URL option discoverable).

[k0smonaut]

Super cool. Thank you for doing this!