Bug fixes and minor enhancements

Fixes:
#1620, #1601

Enhancements:

• new method Page.load_widget() to load a widget from its xref
• new dictionary pdfcolor which contains 500 predefined PDF colors
• Quad class supports operator algebra
• text search and extraction default flags now accessible as predefined constants
• iterators Page.annots() and Page.widgets() now prohibit reloading the page within their scope
• removed multiple utility functions from the Tools class and redefined them as standalone
• Parameter new in Document.update_stream() is now obsolete.