Small Code/Editor cleanup pass (#4909)

* Clean-up in ConfigGroup

Removed unused templated AddVar and related code.
Replaced legacy types with `AZ::` ones.
Cleaned up cpp file.

Signed-off-by: nemerle <96597+nemerle@users.noreply.github.com>

* Add a few missing Q_OBJECT macros, remove some unused variables.

Signed-off-by: nemerle <96597+nemerle@users.noreply.github.com>

* Apply some of clazy suggestions + simplifications

* removed `emit` from non-signal function calls.
* replaced `QStringLiteral("")` with a constexpr friendly
`QLatin1String()`

Signed-off-by: nemerle <96597+nemerle@users.noreply.github.com>

* Fix a CNewLevelDialog focus bug

Fixed an incorrect QTimer::singleShot invocation.

Signed-off-by: nemerle <96597+nemerle@users.noreply.github.com>

* match lambda to `messageChanged` signature

Signed-off-by: nemerle <96597+nemerle@users.noreply.github.com>

* vs compilation fix + applied review

Signed-off-by: nemerle <96597+nemerle@users.noreply.github.com>

* apply reviewer recommendation

Signed-off-by: nemerle <96597+nemerle@users.noreply.github.com>

Code/Editor/NewLevelDialog.cpp

@@ -100,9 +100,10 @@ CNewLevelDialog::CNewLevelDialog(QWidget* pParent /*=nullptr*/)
     m_level = "";
     // First of all, keyboard focus is related to widget tab order, and the default tab order is based on the order in which
     // widgets are constructed. Therefore, creating more widgets changes the keyboard focus. That is why setFocus() is called last.
-    // Secondly, using singleShot() allows setFocus() slot of the QLineEdit instance to be invoked right after the event system
-    // is ready to do so. Therefore, it is better to use singleShot() than directly call setFocus().
-    QTimer::singleShot(0, ui->LEVEL, SLOT(OnStartup()));
+    // in OnStartup()
+    // Secondly, using singleShot() allows OnStartup() slot of the QLineEdit instance to be invoked right after the event system
+    // is ready to do so. Therefore, it is better to use singleShot() than directly call OnStartup().
+    QTimer::singleShot(0, this, &CNewLevelDialog::OnStartup);
 
     ReloadLevelFolder();
 }