[ui] Introduce ClipboardHelper for copying to clipboard from QML

* add ClipboardHelper class that contains a QClipboard and exposes its method as Slots
* use Clipboard instead of hidden TextEdit where meaningful

meshroom/ui/components/clipboard.py

@@ -0,0 +1,20 @@
+from PySide2.QtCore import Slot, QObject
+from PySide2.QtGui import QClipboard
+
+
+class ClipboardHelper(QObject):
+    """
+    Simple wrapper around a QClipboard with methods exposed as Slots for QML use.
+    """
+
+    def __init__(self, parent=None):
+        super(ClipboardHelper, self).__init__(parent)
+        self._clipboard = QClipboard(parent=self)
+
+    @Slot(str)
+    def setText(self, value):
+        self._clipboard.setText(value)
+
+    @Slot()
+    def clear(self):
+        self._clipboard.clear()