Change old slot syntax (removed in vue3) to new one

In addition it fixes some semi-related issues:

• Several places where popover is used trigger wasn't a <button>, changed to button.
  • exception - ModifiedIndicator, since it's not really interactive, used aria-label instead to make it accessible and NOT generate a tooltip.
• Timeline menu (the dropdown home/dms/local/etc switcher) was using panel panel-default classes, switched it to popover-default so that it uses tooltips background (has separate opacity from panels for better visibility)
• Excessive <div>s that don't do anything replaced with <template>s