gdb/python: add PendingFrame.level and Frame.level methods

gdb/python: add PendingFrame.level and Frame.level methods

Add new methods to the PendingFrame and Frame classes to obtain the
stack frame level for each object.

The use of 'level' as the method name is consistent with the existing
attribute RecordFunctionSegment.level (though this is an attribute
rather than a method).

For Frame/PendingFrame I went with methods as these classes currently
only use methods, including for simple data like architecture, so I
want to be consistent with this interface.

gdb/ChangeLog:

	* NEWS: Mention the two new methods.
	* python/py-frame.c (frapy_level): New function.
	(frame_object_methods): Register 'level' method.
	* python/py-unwind.c (pending_framepy_level): New function.
	(pending_frame_object_methods): Register 'level' method.

gdb/doc/ChangeLog:

	* python.texi (Unwinding Frames in Python): Mention
	PendingFrame.level.
	(Frames In Python): Mention Frame.level.

gdb/testsuite/ChangeLog:

	* gdb.python/py-frame.exp: Add Frame.level tests.
	* gdb.python/py-pending-frame-level.c: New file.
	* gdb.python/py-pending-frame-level.exp: New file.
	* gdb.python/py-pending-frame-level.py: New file.