LibGUI: Use default instead of an empty constructor/destructor

Default implementations allow for more optimizations.
See: https://pvs-studio.com/en/docs/warnings/v832/

Userland/Libraries/LibGUI/Margins.h

@@ -10,7 +10,7 @@ namespace GUI {
 
 class Margins {
 public:
-    Margins() { }
+    Margins() = default;
     Margins(int all)
         : m_top(all)
         , m_right(all)
@@ -39,7 +39,7 @@ public:
         , m_left(left)
     {
     }
-    ~Margins() { }
+    ~Margins() = default;
 
     bool is_null() const { return !m_left && !m_top && !m_right && !m_bottom; }
 

Userland/Libraries/LibGUI/ModelIndex.h

@@ -17,7 +17,7 @@ class ModelIndex {
     friend class Model;
 
 public:
-    ModelIndex() { }
+    ModelIndex() = default;
 
     bool is_valid() const { return m_model && m_row != -1 && m_column != -1; }
     int row() const { return m_row; }

Userland/Libraries/LibGUI/Shortcut.h

@@ -13,7 +13,7 @@ namespace GUI {
 
 class Shortcut {
 public:
-    Shortcut() { }
+    Shortcut() = default;
     Shortcut(u8 modifiers, KeyCode key)
         : m_modifiers(modifiers)
         , m_key(key)

Userland/Libraries/LibGUI/TextPosition.h

@@ -12,7 +12,7 @@ namespace GUI {
 
 class TextPosition {
 public:
-    TextPosition() { }
+    TextPosition() = default;
     TextPosition(size_t line, size_t column)
         : m_line(line)
         , m_column(column)