Fixed the bug where button_states() returns None when /AP points …

…to an indirect object.

Description: Some PDF widget objects may not have their `/AP`(Appearance Dictionary) directly pointing to a dictionary, which is enclosed with '<< >>', but rather pointing to an indirect object, expressed by an xref number in the format 'NNN 0 R'. Therefore, the current button_states() only can correctly handle cases where `/AP` points to a dictionary and cannot handle cases where it points to an indirect object. Consequently, I have introduced additional logic to handle the latter scenario, ensuring that `button_states()` can accurately return the on/off state names for button widgets even when `/AP` points to an indirect object.
Test: I have tested the modified `button_states()` function on the mentioned type of PDF, and it now correctly returns the states instead of None. I have copied the method's code to the same-named method in `src/__init__.py`.

fitz/helper-fields.i

@@ -1097,6 +1097,14 @@ class Widget(object):
             for x in apnt:
                 nstates.append(x.split()[0])
             states["normal"] = nstates
+        if APN[0] == "xref":
+            nstates = []
+            nxref = int(APN[1].split(" ")[0])
+            APN = doc.xref_object(nxref)
+            apnt = APN.split("/")[1:]
+            for x in apnt:
+                nstates.append(x.split()[0])
+            states["normal"] = nstates
         APD = doc.xref_get_key(xref, "AP/D")
         if APD[0] == "dict":
             dstates = []
@@ -1105,6 +1113,14 @@ class Widget(object):
             for x in apdt:
                 dstates.append(x.split()[0])
             states["down"] = dstates
+        if APD[0] == "xref":
+            dstates = []
+            dxref = int(APD[1].split(" ")[0])
+            APD = doc.xref_object(dxref)
+            apdt = APD.split("/")[1:]
+            for x in apdt:
+                dstates.append(x.split()[0])
+            states["down"] = dstates
         return states
 
     def on_state(self):

src/__init__.py

@@ -6992,6 +6992,14 @@ def button_states(self):
             for x in apnt:
                 nstates.append(x.split()[0])
             states["normal"] = nstates
+        if APN[0] == "xref":
+            nstates = []
+            nxref = int(APN[1].split(" ")[0])
+            APN = doc.xref_object(nxref)
+            apnt = APN.split("/")[1:]
+            for x in apnt:
+                nstates.append(x.split()[0])
+            states["normal"] = nstates
         APD = doc.xref_get_key(xref, "AP/D")
         if APD[0] == "dict":
             dstates = []
@@ -7000,6 +7008,14 @@ def button_states(self):
             for x in apdt:
                 dstates.append(x.split()[0])
             states["down"] = dstates
+        if APD[0] == "xref":
+            dstates = []
+            dxref = int(APD[1].split(" ")[0])
+            APD = doc.xref_object(dxref)
+            apdt = APD.split("/")[1:]
+            for x in apdt:
+                dstates.append(x.split()[0])
+            states["down"] = dstates
         return states
 
     @property