Small update with some additions, bug fixes and UI changes.

Changelog:

• Added merger simulation mode

• Added exit button

• Worked on the UI's right side

• Added drop down menus for color, simulation modes and materials

• Moved fluid materials to the right side

• Moved "Ship Gas" button to the "Fun" tab in the tools menu

• Fixed camera following dark matter particles

• Particle size now doesn't affect neighbor search