Add Leaderboards tab to Achievement dialog

A new tab is added to the Achievements dialog to chart out the leaderboards in a table. Each row of the table contains the leaderboard information and up to four relevant entries, varying based on how many entries are in the leaderboard, whether or not the player has a submitted score, and where in the leaderboard the player's score is.

Source/Core/DolphinQt/Achievements/AchievementsWindow.h

@@ -10,6 +10,7 @@
 #include "DolphinQt/QtUtils/QueueOnObject.h"
 
 class AchievementHeaderWidget;
+class AchievementLeaderboardWidget;
 class AchievementProgressWidget;
 class QDialogButtonBox;
 class QTabWidget;
@@ -30,6 +31,7 @@ private:
   AchievementHeaderWidget* m_header_widget;
   QTabWidget* m_tab_widget;
   AchievementProgressWidget* m_progress_widget;
+  AchievementLeaderboardWidget* m_leaderboard_widget;
   QDialogButtonBox* m_button_box;
 };