Cheats Manager UX improvements.

[AdmiralCurtiss]

Fixes some annoyances pointed out by @Dentomologist back in #9886.

Specifically:

• The Cheats Manager option is no longer mysteriously greyed out if no game is running or if cheats are disabled and is instead always active.
• If no game is running the code tabs are greyed out instead of completely missing.
• If cheats are disabled and the "Warning: Cheats are disabled! [Configure Dolphin]" header thing shows (as the equivalent tabs in the game properties), the button now actually works -- it didn't before. Granted, it was rather hard to even reach this window in that situation (you'd have to start a game, open the window, close the game, then disable cheats without closing the window), but either way, it works correctly now.
• This also now allows running memory searches even if cheats are disabled, which is just a nice convenience thing.

[iwubcode]

Looks reasonable, untested.