Huge refactor

- Remove some underscore-prefixes
 - Start using this-> more
 - Remove a few of the singletons (We pass references to managers to
         things that need it now. Might not be much better, but for now
         it works. It also shows what places might be slightly wrong
         designed)

src/windowmanager.hpp

@@ -6,13 +6,12 @@
 
 namespace chatterino {
 
+class ChannelManager;
+
 class WindowManager
 {
 public:
-    static WindowManager &getInstance()
-    {
-        return instance;
-    }
+    explicit WindowManager(ChannelManager &_channelManager);
 
     void layoutVisibleChatWidgets(Channel *channel = nullptr);
     void repaintVisibleChatWidgets(Channel *channel = nullptr);
@@ -25,12 +24,12 @@ public:
     void save();
 
 private:
-    static WindowManager instance;
+    ChannelManager &channelManager;
 
-    WindowManager();
+    std::mutex windowMutex;
 
-    std::mutex _windowMutex;
-    widgets::MainWindow *_mainWindow;
+    // TODO(pajlada): Store as a value instead of a pointer
+    widgets::MainWindow *mainWindow = nullptr;
 };
 
 }  // namespace chatterino