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/ErrorDialog.cpp

@@ -25,9 +25,9 @@ namespace SandboxEditor
         connect(m_ui->okButton, &QPushButton::clicked, this, &ErrorDialog::OnOK);
         connect(
             m_ui->messages, 
-            SIGNAL(itemSelectionChanged()), 
+            &QTreeWidget::itemSelectionChanged,
             this, 
-            SLOT(MessageSelectionChanged()));
+            &ErrorDialog::MessageSelectionChanged);
     }
 
     ErrorDialog::~ErrorDialog()