LibGUI: Simplify Dialog::ScreenPosition and allow Dialogs to opt out

These options were created 2 years ago but the only use cases thus far
are for Center and CenterWithinParent, so let's chuck the rest for now.

Adds a new DoNotPosition option to opt out of automatic centering and
a ScreenPosition setter and getter for configuration before exec()

Userland/Libraries/LibGUI/Dialog.h

@@ -22,20 +22,11 @@ public:
         Yes = 3,
         No = 4,
     };
+
     enum class ScreenPosition {
-        CenterWithinParent = 0,
-
-        Center = 1,
-        CenterLeft = 2,
-        CenterRight = 3,
-
-        TopLeft = 4,
-        TopCenter = 5,
-        TopRight = 6,
-
-        BottomLeft = 7,
-        BottomCenter = 8,
-        BottomRight = 9,
+        DoNotPosition,
+        CenterWithinParent,
+        Center,
     };
 
     virtual ~Dialog() override = default;
@@ -45,6 +36,9 @@ public:
     ExecResult result() const { return m_result; }
     void done(ExecResult);
 
+    ScreenPosition screen_position() const { return m_screen_position; }
+    void set_screen_position(ScreenPosition position) { m_screen_position = position; }
+
     virtual void event(Core::Event&) override;
 
     virtual void close() override;