MAVExplorer Multimonitor support problem

Reproduce by dragging a graph window to another monitor, then performing a zoom operation on any graph.
Observed on Win10 and Win11; following is from Win11:

```
Exception in thread Thread-32723:
Traceback (most recent call last):
File "threading.py", line 932, in _bootstrap_inner
File "threading.py", line 1254, in run
File "site-packages\mavproxy-1.8.20-py3.8.egg\MAVProxy\modules\lib\grapher.py", line 465, in xlim_timer
File "site-packages\mavproxy-1.8.20-py3.8.egg\MAVProxy\modules\lib\grapher.py", line 456, in xlim_change_check
File "site-packages\matplotlib\backend_bases.py", line 2965, in push_current
File "site-packages\matplotlib\backend_bases.py", line 2968, in
File "site-packages\matplotlib\axes\_base.py", line 831, in get_position
File "site-packages\matplotlib\axes\_base.py", line 1466, in apply_aspect
File "site-packages\matplotlib\axes\_base.py", line 873, in _set_position
File "site-packages\matplotlib\artist.py", line 228, in stale
File "site-packages\matplotlib\figure.py", line 47, in _stale_figure_callback
File "site-packages\matplotlib\artist.py", line 228, in stale
File "site-packages\matplotlib\pyplot.py", line 597, in _auto_draw_if_interactive
File "site-packages\matplotlib\backends\_backend_tk.py", line 274, in draw_idle
File "tkinter\__init__.py", line 820, in after_idle
File "tkinter\__init__.py", line 811, in after
File "tkinter\__init__.py", line 1519, in _register
RuntimeError: main thread is not in main loop
```

Start in MAVProxy/modules/lib/grapher.py at xlim_timer and xlim_change_check, the functions named in the traceback. Reproduce on Windows by moving a graph window to another monitor and zooming; done means the operation completes without the main-thread RuntimeError.

Written by the indexing model from the issue text.