minor refactoring + features from js vesion

[ker2x]

i added many harmless refactoring, potentially beneficial to performance (many const)

• 1 feature to change the boundary on window resize (when boundary is enabled)
• added gravity feature
• wall repel

[ker2x]

i just added gravity in the cpp code as well since there is a pull request for it on the html code.

[ker2x]

• wall repel (thx @skal65535 )