[TreeView] Add visual progress indicators to domain tiles

On the center/current tile (src/components/TreeView/TreeDepiction/CurrentRow.tsx):

• Small circle in an upper corner with a number of senses with that domain
  • Hover text: "Number of words gathered in this domain"
  • Add function to Backend\Controllers\StatisticsController.cs for getting that number

On the sibling and children tiles (src/components/TreeView/TreeDepiction/DomainTileButton.tsx):

• Not on the parent tile (so only for Direction.Down, .Prev, .Next)
• A thin progress bar along the bottom edge showing proportion of descendent domains that have at least one entry
  • Same color as the border
  • Empty will just look like a second line along the bottom, with a very narrow gap between it at the bottom border
  • Full will just look like a thick bottom border
  • Add function to Backend\Controllers\StatisticsController.cs for getting that proportion