[Python] Inject gPad and gVirtualX into facade without ROOT meta

The `gPad and `gVirtualX` identifiers are injected into ROOT meta via
`TGlobalMappedFunction::MakeFunctor()`. However, in the ROOT Python
interface we don't want to rely on ROOT meta but use Cling or Clang-Repl
directly. Hence, we inject these identifiers manually into the facade.

The `TDirectoryPythonAdapter` previously used for `gDirectory` mirrored
the live-tracking semantics of the C++ macros: every attribute access
re-resolves to the current directory. The same semantics are needed
for `gPad` and `gVirtualX`, which on the C++ side are also preprocessor
macros that expand to a static accessor call. Therefore, a new
`LiveProxy` is introduced for this.

Author: guitargeek

bindings/pyroot/pythonizations/python/ROOT/_facade.py

@@ -305,7 +305,6 @@ def _finalSetup(self):
 
         # Make sure the interpreter is initialized once gROOT has been initialized
         self.__dict__["gInterpreter"] = self._cppyy.gbl.TInterpreter.Instance()
-        self.__dict__["gPad"] = self._cppyy.gbl.TVirtualPad.Pad()
 
         # Release the GIL on the heavy TInterpreter functions. This lets
         # background Python threads make progress - in particular, JupyROOT's