Feature Friday - 2019.9.13 — Umbraseal Steam update #1417644

The scenario system almost ready. But before we add story, we still want to adjust difficulty and game flow. We tend to make Umbraseal more friendly for beginner. So we divide difficulty to four level. The enemy in easier difficulty will shoot less bullets or be spawned in smaller group. In addition, we may replace level selection by more linear game flow to decrease play time in each stage. Feature adjust enemy hp by difficulty. adjust seal number by difficulty. adjust map size by difficulty. adjust minion bullet pattern by difficulty. reduce boss stage phase by difficulty. adjust boss bullet pattern by difficulty. town and library normal level add middle stage. adjust enemy pack size by difficulty. make all level available.