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/main/rels/ClassWrapper.java

@@ -123,7 +123,7 @@ public class ClassWrapper {
           }
         }
         else {
-          boolean thisvar = (mt.getAccessFlags() & CodeConstants.ACC_STATIC) == 0;
+          boolean thisvar = !mt.hasModifier(CodeConstants.ACC_STATIC);
           MethodDescriptor md = MethodDescriptor.parseDescriptor(mt.getDescriptor());
 
           int paramcount = 0;