LibGUI: Add DialogButton convenience class

DialogButton is a small convenience class, that just gives a button to
be used in dialogs for a more consistent look around the system.

Userland/Libraries/LibGUI/Button.cpp

@@ -16,6 +16,7 @@
 #include <LibGfx/StylePainter.h>
 
 REGISTER_WIDGET(GUI, Button)
+REGISTER_WIDGET(GUI, DialogButton)
 
 namespace GUI {
 

Userland/Libraries/LibGUI/Button.h

@@ -76,4 +76,16 @@ private:
     bool m_mimic_pressed { false };
 };
 
+class DialogButton final : public Button {
+    C_OBJECT(DialogButton);
+
+public:
+    virtual ~DialogButton() override {};
+    explicit DialogButton(String text = {})
+        : Button(move(text))
+    {
+        set_fixed_width(80);
+    }
+};
+
 }

Userland/Libraries/LibGUI/Forward.h

@@ -23,6 +23,7 @@ class CheckBox;
 class ComboBox;
 class Command;
 class CommandPalette;
+class DialogButton;
 class DragEvent;
 class DropEvent;
 class EditingEngine;