Sketch code for gateway lookup algorithm. I decided to keep it as Igniter part, since it's the tool which in fact performs gateway activation.
Massive changes

1. Portal invisible block now has second function of 'shield' block which forms around newly created exit in Nether and protects from lava. It might have anti-projectile function at some point
2. DimensionManager dropped
3. Gateway tile now doesn't perform any kind of search for destination
4. Gate igniter needs massive rework and cleanup
Major simplifications

1. No more separate igniter; this function should be maintained by standard flint'n'steel. And no more requirements for ender pearls.
2. Gateway tile now contains owner username. This is used to allow gateway removal only for the one who installed it.
3. Moved full gateway establishment code to utils.