Perform initial refactoring work

Things that were once singletons are no longer singletons, but are
instead stored in the "Application" singleton

Some singletons still remain, and some renaming/renamespacing is left

src/widgets/settingspages/accountspage.cpp

@@ -1,13 +1,14 @@
 #include "accountspage.hpp"
 
-#include <QDialogButtonBox>
-#include <QVBoxLayout>
-
+#include "application.hpp"
 #include "const.hpp"
 #include "singletons/accountmanager.hpp"
 #include "util/layoutcreator.hpp"
 #include "widgets/logindialog.hpp"
 
+#include <QDialogButtonBox>
+#include <QVBoxLayout>
+
 namespace chatterino {
 namespace widgets {
 namespace settingspages {
@@ -39,7 +40,7 @@ AccountsPage::AccountsPage()
             return;
         }
 
-        singletons::AccountManager::getInstance().Twitch.removeUser(selectedUser);
+        getApp()->accounts->Twitch.removeUser(selectedUser);
     });
 }