Emit progress events from VtkLocal and document global loader usage

[abhinavsns]

See Issue: #49
Linked PR: Kitware/vtk-wasm#20
Wire vtk-wasm progress into VtkLocal with an overlay during wasm load/sync; emit a progress event from the Vue component; expose progress in the Python widget and document how to drive a global app loader.
Usage (Python/trame global loader):

from trame.widgets import vtklocal, vuetify

state.app_loading = True
state._vtklocal_seen_active = False

def on_vtklocal_progress(event):
    if event.get("active"):
        state._vtklocal_seen_active = True
        state.app_loading = True
    elif state._vtklocal_seen_active:
        state.app_loading = False

with vuetify.VOverlay(v_model=("app_loading",), absolute=True):
    vuetify.VProgressCircular(indeterminate=True, size=64)

vtklocal.LocalView(render_window, progress=(on_vtklocal_progress, "[$event]"))

Usage (Vue):
<VtkLocal @progress="onProgress" />

[abhinavsns]

@jspanchu I have reverted the build_vtk.py file.

[jspanchu]

@abhinavsns Thanks. LGTM

[jourdain]

Now that @kitware/vtk-wasm has been released with your changes, it might be good to update the package.json + lock to bring that feature into the pip package once the ci/release run.

[jspanchu]

LGTM. can you bump the version of vtk-wasm in package.json and the lock file as well?

[abhinavsns]

Done!