Enhanced ui.avatar, more documentation, and bugfixes

Improvements

• Allow images inside ui.avatar (#903 by @rodja)

Documentation Updates

• Fixed type hint for set_visibility() method (#911 by @jonathanunderwood and @rodja)
• New demo showing multiline notifications (#914 by @rodja)
• New demo showing how to add rows (#913 by @rodja)

Bug Fixes

• Special characters like % can't be pushed to ui.log (#899 by @falkoschindler and @phoskee)
• element.move() does not work as expected in some cases (#898 by @falkoschindler)