LibGUI: Add a GAbstractButton base class for button widgets.

This patch moves GButton and GRadioButton to inherit from it. This allows
them to share code for mouse event handling, etc.

LibGUI/GAbstractButton.cpp

@@ -0,0 +1,106 @@
+#include <LibGUI/GAbstractButton.h>
+
+GAbstractButton::GAbstractButton(GWidget* parent)
+    : GWidget(parent)
+{
+}
+
+GAbstractButton::GAbstractButton(const String& text, GWidget* parent)
+    : GWidget(parent)
+    , m_text(text)
+{
+}
+
+GAbstractButton::~GAbstractButton()
+{
+}
+
+void GAbstractButton::set_text(const String& text)
+{
+    if (m_text == text)
+        return;
+    m_text = text;
+    update();
+}
+
+void GAbstractButton::set_checked(bool checked)
+{
+    if (m_checked == checked)
+        return;
+    m_checked = checked;
+    update();
+}
+
+void GAbstractButton::set_checkable(bool checkable)
+{
+    if (m_checkable == checkable)
+        return;
+    m_checkable = checkable;
+    update();
+}
+
+void GAbstractButton::mousemove_event(GMouseEvent& event)
+{
+    bool is_over = rect().contains(event.position());
+    m_hovered = is_over;
+    if (event.buttons() & GMouseButton::Left) {
+        if (is_enabled()) {
+            bool being_pressed = is_over;
+            if (being_pressed != m_being_pressed) {
+                m_being_pressed = being_pressed;
+                update();
+            }
+        }
+    }
+    GWidget::mousemove_event(event);
+}
+
+void GAbstractButton::mousedown_event(GMouseEvent& event)
+{
+#ifdef GABSTRACTBUTTON_DEBUG
+    dbgprintf("GAbstractButton::mouse_down_event: x=%d, y=%d, button=%u\n", event.x(), event.y(), (unsigned)event.button());
+#endif
+    if (event.button() == GMouseButton::Left) {
+        if (is_enabled()) {
+            m_being_pressed = true;
+            update();
+        }
+    }
+    GWidget::mousedown_event(event);
+}
+
+void GAbstractButton::mouseup_event(GMouseEvent& event)
+{
+#ifdef GABSTRACTBUTTON_DEBUG
+    dbgprintf("GAbstractButton::mouse_up_event: x=%d, y=%d, button=%u\n", event.x(), event.y(), (unsigned)event.button());
+#endif
+    if (event.button() == GMouseButton::Left) {
+        if (is_enabled()) {
+            bool was_being_pressed = m_being_pressed;
+            m_being_pressed = false;
+            update();
+            if (was_being_pressed)
+                click();
+        }
+    }
+    GWidget::mouseup_event(event);
+}
+
+void GAbstractButton::enter_event(CEvent&)
+{
+    m_hovered = true;
+    update();
+}
+
+void GAbstractButton::leave_event(CEvent&)
+{
+    m_hovered = false;
+    update();
+}
+
+void GAbstractButton::keydown_event(GKeyEvent& event)
+{
+    if (event.key() == KeyCode::Key_Return)
+        click();
+    GWidget::keydown_event(event);
+}