java-decompiler: fixes and cleanups

- unified attribute loading code
- common methods for checking member flags
- verifying skip()
- correct resource closing
- typos

src/org/jetbrains/java/decompiler/struct/gen/DataPoint.java

@@ -65,7 +65,7 @@ public class DataPoint {
     MethodDescriptor md = MethodDescriptor.parseDescriptor(mt.getDescriptor());
 
     int k = 0;
-    if ((mt.getAccessFlags() & CodeConstants.ACC_STATIC) == 0) {
+    if (!mt.hasModifier(CodeConstants.ACC_STATIC)) {
       point.setVariable(k++, new VarType(CodeConstants.TYPE_OBJECT, 0, null));
     }