LibGUI: Disable increment/decrement buttons on SpinBox based on value

When the value for a SpinBox equals the max, disable the increment
button. Functionally, clicking the button doesn't do anything because
the set_value() clamps the value to min/max and updates the textbox.
However it is still nice to indicate to the user that they've reached
the max. Same goes for minimum value and the decrement button.

Userland/Libraries/LibGUI/SpinBox.cpp

@@ -69,6 +69,10 @@ void SpinBox::set_value(int value, AllowCallback allow_callback)
     if (m_value == value)
         return;
     m_value = value;
+
+    m_increment_button->set_enabled(m_value < m_max);
+    m_decrement_button->set_enabled(m_value > m_min);
+
     m_editor->set_text(String::number(value));
     update();
     if (on_change && allow_callback == AllowCallback::Yes)