Diffie-Hellman Key Exchange revision

[grthor]

Buttons removed from plugin header:

• The button to switch the animation on and off is now in the menubar.
• The buttons for saving are now under the key exchange visualization. Added monospaced font to key textfields.

Online help in German and English extended with the animation off button. Updated screenshots. Content of the manual revised.

This is how the plugin looked before:

This is what the plugin looks like now: