ENH: Add visible close button in the top-right corner (#28)

* feat: Add close button and unit test for efck#26

- Adds an 'X' button to the top-right corner of the main window for closing the application.
- Adds a unit test verifying the close button triggers application quit.
- Updates Qt abstraction layer to ensure QPushButton is available.
- Refines test setup with necessary mocks for QApplication.instance.

* Revert complex test

* Fixup

* Update efck/gui.py

* Update efck/gui.py

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: kernc

efck/_qt/pyqt5.py

@@ -56,6 +56,7 @@
     QListView,
     QListWidget,
     QListWidgetItem,
+    QPushButton,
     QScrollArea,
     QSizeGrip,
     QSlider,

efck/_qt/pyqt6.py

@@ -56,6 +56,7 @@
     QListView,
     QListWidget,
     QListWidgetItem,
+    QPushButton,
     QScrollArea,
     QSizeGrip,
     QSlider,

efck/_qt/pyside6.py

@@ -59,6 +59,7 @@
     QListView,
     QListWidget,
     QListWidgetItem,
+    QPushButton,
     QScrollArea,
     QSizeGrip,
     QSlider,

efck/_qt/qtpy.py

@@ -56,6 +56,7 @@
     QListView,
     QListWidget,
     QListWidgetItem,
+    QPushButton,
     QScrollArea,
     QSizeGrip,
     QSlider,

efck/gui.py

@@ -151,6 +151,15 @@ def __init__(self, parent):
         corner_widget = CornerWidget(self)
         self.setCornerWidget(corner_widget, Qt.Corner.TopLeftCorner)
 
+        close_button = QPushButton(
+            icon=self.style().standardIcon(QStyle.StandardPixmap.SP_TitleBarCloseButton),
+            flat=True,
+            toolTip='Close',
+            parent=self,
+        )
+        close_button.clicked.connect(QApplication.instance().quit)
+        self.setCornerWidget(close_button, Qt.Corner.TopRightCorner)
+
         # Populate tabs
 
         from .tab import Tab