Core: Remove some header inclusions in header files

Replaces them with forward declarations of used types, or removes them entirely if they aren't used at all. This also replaces certain Common headers with less inclusive ones (in terms of definitions they pull in).

Source/Core/Core/Boot/Boot.cpp

@@ -12,6 +12,7 @@
 
 #include "Core/ConfigManager.h"
 #include "Core/Core.h"
+#include "Core/CoreParameter.h"
 #include "Core/Host.h"
 #include "Core/PatchEngine.h"
 #include "Core/Boot/Boot.h"

Source/Core/Core/Boot/Boot.h

@@ -7,10 +7,7 @@
 #include <cstdlib>
 #include <string>
 
-#include "Core/CoreParameter.h"
-
 #include "DiscIO/Volume.h"
-using DiscIO::IVolume;
 
 struct CountrySetting
 {
@@ -58,5 +55,5 @@ private:
 	static bool Load_BS2(const std::string& _rBootROMFilename);
 	static void Load_FST(bool _bIsWii);
 
-	static bool SetupWiiMemory(IVolume::ECountry country);
+	static bool SetupWiiMemory(DiscIO::IVolume::ECountry country);
 };

Source/Core/Core/Boot/Boot_BS2Emu.cpp

@@ -163,18 +163,18 @@ bool CBoot::EmulatedBS2_GC(bool skipAppLoader)
 	return true;
 }
 
-bool CBoot::SetupWiiMemory(IVolume::ECountry country)
+bool CBoot::SetupWiiMemory(DiscIO::IVolume::ECountry country)
 {
 	static const CountrySetting SETTING_EUROPE = {"EUR", "PAL",  "EU", "LE"};
-	static const std::map<IVolume::ECountry, const CountrySetting> country_settings = {
-		{IVolume::COUNTRY_EUROPE, SETTING_EUROPE},
-		{IVolume::COUNTRY_USA,    {"USA", "NTSC", "US", "LU"}},
-		{IVolume::COUNTRY_JAPAN,  {"JPN", "NTSC", "JP", "LJ"}},
-		{IVolume::COUNTRY_KOREA,  {"KOR", "NTSC", "KR", "LKH"}},
+	static const std::map<DiscIO::IVolume::ECountry, const CountrySetting> country_settings = {
+		{DiscIO::IVolume::COUNTRY_EUROPE, SETTING_EUROPE},
+		{DiscIO::IVolume::COUNTRY_USA,    {"USA", "NTSC", "US", "LU"}},
+		{DiscIO::IVolume::COUNTRY_JAPAN,  {"JPN", "NTSC", "JP", "LJ"}},
+		{DiscIO::IVolume::COUNTRY_KOREA,  {"KOR", "NTSC", "KR", "LKH"}},
 		//TODO: Determine if Taiwan have their own specific settings.
 		//      Also determine if there are other specific settings
 		//      for other countries.
-		{IVolume::COUNTRY_TAIWAN, {"JPN", "NTSC", "JP", "LJ"}}
+		{DiscIO::IVolume::COUNTRY_TAIWAN, {"JPN", "NTSC", "JP", "LJ"}}
 	};
 	auto entryPos = country_settings.find(country);
 	const CountrySetting& country_setting =