Various widget improvements

[mmstick]

Fixed alignment of menu buttons, and uses icons instead of unicode text

Improvements for the Grid widget:

• Fixes padding not being applied correctly
• Fixes widget using incorrect width and height for its layout
  • It's now possible to add it to a container and center with container.center_x()
• Adds justify_content property
• Sets flex_grow when a widget is Fill

Improvements for the FlexRow widget:

• Rewrites layout with taffy, as used by the grid widget
• Adds align_items, justify_items, and justify_content properties
• Adds min_item_width property for wrapping text widgets set to Fill

Adds flex_ variants of settings::item, settings::item_row, and settings::Item::control

Other improvements

• Added re-exports for the Canvas and TextEdit widgets from Iced
• Removed 'static lifetime restriction from icon widget
• menu tree's menu_button is now a function instead of a macro