Kernel: Set correct KeyCode count

The underlying value of the `KeyCode::Key_*` enum values starts at 0,
so we should add 1 to `Key_Menu` to get the correct count.

Kernel/API/KeyCode.h

@@ -152,7 +152,7 @@ enum KeyCode : u8 {
         Key_Shift
     = Key_LeftShift,
 };
-int const key_code_count = Key_Menu;
+size_t const key_code_count = Key_Menu + 1;
 
 enum KeyModifier {
     Mod_None = 0x00,

Userland/Applications/Piano/MainWidget.cpp

@@ -189,7 +189,7 @@ void MainWidget::turn_off_pressed_keys()
 {
     if (m_keys_widget->mouse_note() != -1)
         m_track_manager.keyboard()->set_keyboard_note_in_active_octave(m_keys_widget->mouse_note(), DSP::Keyboard::Switch::Off);
-    for (int i = 0; i < key_code_count; ++i) {
+    for (size_t i = 0u; i < key_code_count; ++i) {
         if (m_keys_pressed[i])
             note_key_action(i, DSP::Keyboard::Switch::Off);
     }
@@ -199,7 +199,7 @@ void MainWidget::turn_on_pressed_keys()
 {
     if (m_keys_widget->mouse_note() != -1)
         m_track_manager.keyboard()->set_keyboard_note_in_active_octave(m_keys_widget->mouse_note(), DSP::Keyboard::Switch::On);
-    for (int i = 0; i < key_code_count; ++i) {
+    for (size_t i = 0u; i < key_code_count; ++i) {
         if (m_keys_pressed[i])
             note_key_action(i, DSP::Keyboard::Switch::On);
     }