Do not retain snake mode layout when passing though snake mode

[ljubica-milovic]

Background

Similar to the recently introduced functionality regarding sequence mode 5372 - if the user just passes though the mode, but does not make any changes the layout should not be permanent.

Requirements

1. If the user enters the snake layout mode, but does not make any changes to the structure before entering some other mode (flex, sequence, small molecules) the snake layout should not be retained.

Changes encompass any addition or deletion of monomers, bonds and small molecules.

2. The user should be able to pass though both snake and sequence mode one after the other and if no changes were made in any of them, previous custom layout should remain upon going back to flex/small molecules mode.