Cheats Manager quality of life improvements.

[AdmiralCurtiss]

See https://forums.dolphin-emu.org/Thread-hexadecimal-searches-cheat-manager

• Geometry of the window is now saved across sessions.
• There's an extra checkbox to force parsing the input value as hexadecimal even without the 0x prefix.
• Starting a new search with standard regions when no game is running is disallowed now, as it could lead to incorrect memory regions being used (GC/Wii difference; also starting before the first game boot returned a memory size of 0).

[iwubcode]

Code looked good to me. One comment that I probably missed before, CheatSearchSession could be marked final for a potentially slight perf boost.

[leoetlino]

looks sane