Explosion blast added and working on the campaign

Updates:

Gameplay:

- Explosions now have a blast effect added and its radius comes from the enemies' configurations.

- If a built object is within this blast radius, it gets damaged by the amount specified in the enemy's config.

UI:

- Skirmish and Campaign separated in Main menu. Skirmish will keep the environment and day cycle selectors, but these fights will be single fights. Campaign does not have these selectors, but both of them have the Tutorial Map option selectable.