Fix for incompatible method call regarding IDEA <2018.2.

Heavy refactoring regarding quick fixes (using more extensions).
Even more refactoring.
Fixed/extended AssertThatGuavaOptionalInspection regarding static.
imports and wrong reference to core assertThat() where Guava assertThat() actually was necessary.

src/test/resources/inspections/AssertThatGuavaOptional/WithoutPriorGuavaImportAfter.java

@@ -0,0 +1,27 @@
+import com.google.common.base.Optional;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.guava.api.Assertions.assertThat;
+
+public class AssertThatGuavaOptional {
+
+    private void assertThatGuavaOptional() {
+        Optional<String> opt = Optional.absent();
+
+        assertThat(opt).contains("foo");
+        assertThat(opt).contains("foo");
+        assertThat(opt).isNotEqualTo(Optional.of("foo"));
+        assertThat(opt).isNotEqualTo(Optional.fromNullable("foo"));
+
+        assertThat(opt).isPresent();
+        assertThat(opt).isAbsent();
+
+        assertThat(opt).contains("foo");
+        assertThat(opt.get()).isSameAs("foo");
+        assertThat(opt.get()).isNotEqualTo("foo");
+        assertThat(opt.get()).isNotSameAs("foo");
+
+        assertThat(opt).isAbsent();
+        assertThat(opt).isPresent();
+    }
+}