Reduce MINI_GAP between participants

[MrCoder]

This card is to revert this commit: 861c706.

The above commit increases the minimum direct gap between Participants from an aesthetic standpoint. Horizontal space is a very scarce resource in sequence diagrams compared to vertical space. Our layout algorithm should try to minimize the horizontal width as much as possible. Only when the number of Participants is less than or equal to three, should consider enhancing the aesthetic by increasing the width.