Refactor color handling

Start moving drawing preferences (colors, widths) to a single file.

Refactor applying them.

b_asic/scheduler_gui/_preferences.py

+from qtpy.QtCore import Qt
+from qtpy.QtGui import QColor
+
+SIGNAL_INACTIVE = QColor(Qt.black)
+SIGNAL_ACTIVE = QColor(Qt.red)
+SIGNAL_WIDTH = 0.03
+
+OPERATION_LATENCY_INACTIVE = QColor(Qt.lightGray)
+OPERATION_LATENCY_ACTIVE = QColor(Qt.red)
+OPERATION_EXECUTION_TIME_INACTIVE = QColor(Qt.magenta)
+OPERATION_EXECUTION_TIME_ACTIVE = QColor(Qt.magenta)