Split up Window Layout loading into a loading and application stage (#1964)

* Split up Window Layout loading into a loading and application stage

Previously, we were creating UI elements at while we were reading the window-layout.json file.
We now read the window-layout.json file fully first, which results in a
WindowLayout struct which is built up of a list of windows with a list
of tabs with a root node which contains containers and splits.
This WindowLayout can then be applied.

This will enable PRs like #1940 to start Chatterino with Window Layouts
that aren't defined in a json file.

This commit has deprecated loading of v1 window layouts (we're now on v2). If a v1 window layout is there, it will just be ignored and Chatterino will boot up as if it did not have a window layout at all, and on save that old window layout will be gone.

* Fix compile error for mac

src/common/WindowDescriptors.hpp

@@ -0,0 +1,97 @@
+#pragma once
+
+#include <QString>
+
+#include <optional>
+#include <variant>
+
+namespace chatterino {
+
+/**
+ * A WindowLayout contains one or more windows.
+ * Only one of those windows can be the main window
+ *
+ * Each window contains a list of tabs.
+ * Only one of those tabs can be marked as selected.
+ *
+ * Each tab contains a root node.
+ * The root node is either a:
+ *  - Split Node (for single-split tabs), or
+ *  - Container Node (for multi-split tabs).
+ *    This container node would then contain a list of nodes on its own, which could be split nodes or further container nodes
+ **/
+
+// from widgets/Window.hpp
+enum class WindowType;
+
+struct SplitDescriptor {
+    // twitch or mentions or watching or whispers or irc
+    QString type_;
+
+    // Twitch Channel name or IRC channel name
+    QString channelName_;
+
+    // IRC server
+    int server_{-1};
+
+    // Whether "Moderation Mode" (the sword icon) is enabled in this split or not
+    bool moderationMode_{false};
+
+    static void loadFromJSON(SplitDescriptor &descriptor,
+                             const QJsonObject &root, const QJsonObject &data);
+};
+
+struct SplitNodeDescriptor : SplitDescriptor {
+    qreal flexH_ = 1;
+    qreal flexV_ = 1;
+};
+
+struct ContainerNodeDescriptor;
+
+using NodeDescriptor =
+    std::variant<ContainerNodeDescriptor, SplitNodeDescriptor>;
+
+struct ContainerNodeDescriptor {
+    qreal flexH_ = 1;
+    qreal flexV_ = 1;
+
+    bool vertical_ = false;
+
+    std::vector<NodeDescriptor> items_;
+};
+
+struct TabDescriptor {
+    QString customTitle_;
+    bool selected_{false};
+    bool highlightsEnabled_{true};
+
+    std::optional<NodeDescriptor> rootNode_;
+};
+
+struct WindowDescriptor {
+    enum class State {
+        None,
+        Minimized,
+        Maximized,
+    };
+
+    WindowType type_;
+    State state_ = State::None;
+
+    QRect geometry_;
+
+    std::vector<TabDescriptor> tabs_;
+};
+
+class WindowLayout
+{
+public:
+    static WindowLayout loadFromFile(const QString &path);
+
+    // A complete window layout has a single emote popup position that is shared among all windows
+    QPoint emotePopupPos_;
+
+    std::vector<WindowDescriptor> windows_;
+};
+
+}  // namespace chatterino