(ui,)settings: Use explicit instantiation

Reduces compile times a tad on clang.

src/common/settings_common.h

@@ -0,0 +1,78 @@
+#pragma once
+
+#include <forward_list>
+#include <functional>
+#include <map>
+#include <string>
+#include <typeindex>
+#include "common/common_types.h"
+
+namespace Settings {
+
+enum class Category : u32 {
+    Audio,
+    Core,
+    Cpu,
+    CpuDebug,
+    CpuUnsafe,
+    Renderer,
+    RendererAdvanced,
+    RendererDebug,
+    System,
+    SystemAudio,
+    DataStorage,
+    Debugging,
+    DebuggingGraphics,
+    Miscellaneous,
+    Network,
+    WebService,
+    AddOns,
+    Controls,
+    Ui,
+    UiGeneral,
+    UiLayout,
+    UiGameList,
+    Screenshots,
+    Shortcuts,
+    Multiplayer,
+    Services,
+    Paths,
+    MaxEnum,
+};
+
+class BasicSetting {
+protected:
+    explicit BasicSetting() = default;
+
+public:
+    virtual ~BasicSetting() = default;
+
+    virtual Category Category() const = 0;
+    virtual constexpr bool Switchable() const = 0;
+    virtual std::string ToString() const = 0;
+    virtual std::string ToStringGlobal() const;
+    virtual void LoadString(const std::string& load) = 0;
+    virtual std::string Canonicalize() const = 0;
+    virtual const std::string& GetLabel() const = 0;
+    virtual std::string DefaultToString() const = 0;
+    virtual bool Save() const = 0;
+    virtual std::type_index TypeId() const = 0;
+    virtual constexpr bool IsEnum() const = 0;
+    virtual bool RuntimeModfiable() const = 0;
+    virtual void SetGlobal(bool global) {}
+    virtual constexpr u32 Id() const = 0;
+    virtual std::string MinVal() const = 0;
+    virtual std::string MaxVal() const = 0;
+    virtual bool UsingGlobal() const;
+};
+
+class Linkage {
+public:
+    explicit Linkage(u32 initial_count = 0);
+    ~Linkage();
+    std::map<Category, std::forward_list<BasicSetting*>> by_category{};
+    std::vector<std::function<void()>> restore_functions{};
+    u32 count;
+};
+
+} // namespace Settings