refactor list and menu items

[codingpaula]

This PR updates the existing list item to be used with the menu or a list and with more features:

• The menu and the context_menu get a new parameter auto_close that previously was given to the menu_item.
• The menu_item element is deprecated in favor of the updated item.
• The updated item can now be given a text which comes in handy when creating simple lists that don't need the functionality provided by item_section and item_label. Initializing an item with text, adds a q-item-section with the text under the hood for better styling.
• The item_section can also receive a text as a parameter. This aligns its functionality more closely to Quasar's item_section.
• The item, item_section and item_label are moved to their own file.
• The documentation and tests are updated to use the new item and item_section consistently.