Core/ConfigManager: Use forward declarations where applicable

Avoids dragging in IniFile, EXI device and SI device headers in this header which is
quite widely used throughout the codebase.

This also uncovered a few cases where indirect inclusions were being
relied upon, which this also fixes.

Source/Core/Common/IniFile.cpp

@@ -121,6 +121,10 @@ bool IniFile::Section::GetLines(std::vector<std::string>* lines, const bool remo
 
 // IniFile
 
+IniFile::IniFile() = default;
+
+IniFile::~IniFile() = default;
+
 const IniFile::Section* IniFile::GetSection(const std::string& sectionName) const
 {
   for (const Section& sect : sections)

Source/Core/Common/IniFile.h

@@ -86,6 +86,9 @@ public:
     std::vector<std::string> m_lines;
   };
 
+  IniFile();
+  ~IniFile();
+
   /**
    * Loads sections and keys.
    * @param filename filename of the ini file which should be loaded