Working on the Meteor shower and some more...

Main update:

- The main focus of work currently is the implementation of the Meteor Shower. It almost fullly works now, but it does not yet damage buildings.

Minor updates:

- Camera movement and rotation sensitivity by mouse and keyboard also implemented.

- Turrets health increased to 30 to match their maximum level.

- Main menu panelts will receive pictograms above the texts.

- Steam username is retrieved every time the user enters the main menu. This was needed in case the player starts the game without Steam running and starting it after the game already done the initialization.