Pacing, gui and effects update

After comparing this game to other games in the genre, I got some new ideas, how I can improve it. So that's what i did:

• Added a button to start the next wave early. This will improve pacing. Before this, the game could feel too slow. Now the player can choose their own pace.
• Replaced star-reached-menu with achievement-effect. The old way the game would interrupt and show a dialog, if the player achieved a star. Now there will be a text on the bottom right that doesn't interrupt the game.
• Added a high score for every level. The game will now remember the highest Wave beaten and show it.
• Added Ground-effect, after enemies get destroyed.