Put all constant definitions in a new constants.py file

This will improve maintainability. For now, the constants are scatered across several files:

$ grep -P "^ *[A-Z_]+ =" vicodepy/*.py
vicodepy/about.py:REPOSITORY_URL = (
vicodepy/about.py:PYPI_URL = "https://pypi.org/project/vicodepy/"
vicodepy/app.py:    DEFAULT_WIDTH = 640
vicodepy/app.py:    DEFAULT_HEIGHT = 480
vicodepy/config.py:CONFIG_FILENAME = "config.yml"
vicodepy/config.py:APP_NAME = "vicodepy"
vicodepy/cursor.py:    PEN_COLOR = Qt.GlobalColor.black
vicodepy/cursor.py:    BRUSH_COLOR = QColor(0, 0, 0, 100)
vicodepy/cursor.py:    HANDLE_HEIGHT = 25
vicodepy/cursor.py:    HANDLE_WIDTH = 10
vicodepy/event.py:    DEFAULT_COLOR = QColor(255, 255, 255)
vicodepy/files.py:    CSV_HEADERS = ["timeline", "event", "begin", "end", "comment"]
vicodepy/files.py:                DEFAULT_COLOR = QColor(255, 255, 255)
vicodepy/format.py:FORMAT = 3
vicodepy/mainwindow.py:    MINIMUM_WIDTH = 640
vicodepy/mainwindow.py:    MINIMUM_HEIGHT = 320
vicodepy/occurrence.py:    DEFAULT_PEN_COLOR = QColor(0, 0, 0)
vicodepy/occurrence.py:    DEFAULT_BG_COLOR = QColor(128, 128, 128)
vicodepy/occurrence.py:    DEFAULT_FONT_COLOR = QColor(0, 0, 0)
vicodepy/occurrence.py:    PEN_WIDTH_ON_CURSOR = 3
vicodepy/occurrence.py:    PEN_WIDTH_OFF_CURSOR = 1
vicodepy/occurrence.py:    PEN_WIDTH_ON = 3
vicodepy/occurrence.py:    PEN_WIDTH_OFF = 1
vicodepy/occurrence.py:    HANDLE_WIDTH = 9
vicodepy/timeline.py:    FIXED_HEIGHT = 20
vicodepy/timepane.py:    MARGIN_BOTTOM = 15.0

There are other constants, which are now hardcoded, that can be transformed into constants.