Added gateway igniter - used to construct gateways from placed keystones
1. Made gateway block and portal block indestructible, changed gateway's texture

2. Added texture for keystone
1. Added keystone block and recipe for it

2. Added recipe for Gateway Igniter
3. Moved Assets to ModMain file
Added logic to igniter - to enable/disable gateway, a player should click gateway base block twice within 10 seconds;

Actual transformation isn't implemented yet
Added code to transform keystone into portal and back via igniter
Added gateway's tile entity and moved all teleportation logic there
Refactoring. Portal management moved to TileGateway. Block scan moved to utils.