Minor UI-related updates

UI:

- Buttons at the bottom UI now have their names specified.

- Buttons now have a tooltip that appears above the build UI when the cursor is over them.

Game mechanics:

- The game now ends if the tower reaches 0 health and a summary screen appears blocking all further contact to the ingame objects and it will show the summary of the whole game (e.g. type of damage dealt, number of buildings of type X built , etc.). The continue game button is disabled after this forcing the player to start a new game instead of allowing him to continue playing on the already ended game.

- When game ends the background objects also stop updating.