Add color button (for later use in preferences)

Related to #104

b_asic/gui_utils/color_button.py

+"""
+Qt button for use in preference dialogs, selecting color.
+"""
+from qtpy.QtCore import Qt, Signal
+from qtpy.QtGui import QColor
+from qtpy.QtWidgets import QColorDialog, QPushButton
+
+
+class ColorButton(QPushButton):
+    """
+    Button to select color.
+
+    Parameters
+    ----------
+    color : QColor
+        The initial color of the button.
+    *args, **kwargs
+        Additional arguments are passed to QPushButton.
+    """
+
+    _color_changed = Signal(QColor)
+
+    def __init__(self, color: QColor, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+        self._color = None
+        self._default = color
+        self.pressed.connect(self.pick_color)
+
+        # Set the initial/default state.
+        self.set_color(self._default)
+
+    def set_color(self, color: QColor):
+        """Set new color."""
+        if color != self._color:
+            self._color = color
+            self._color_changed.emit(color)
+
+        if self._color:
+            self.setStyleSheet("background-color: %s;" % self._color)
+        else:
+            self.setStyleSheet("")
+
+    @property
+    def color(self):
+        """Current color."""
+        return self._color
+
+    def pick_color(self):
+        """Show color-picker dialog to select color."""
+        dlg = QColorDialog(self)
+        if self._color:
+            dlg.setCurrentColor(self._color)
+
+        if dlg.exec_():
+            self.set_color(dlg.currentColor())
+
+    def mousePressEvent(self, e):
+        if e.button() == Qt.RightButton:
+            self.set_color(self._default)
+
+        return super().mousePressEvent(e)