[gsoc] support open/save/copy/paste/new actions in gr-web

[eat4toast]

This draft pr intend to support open/save/copy/paste/new actions in gnuradio-web
those contexts are still WIP since there are some weird interactions with emscripten virtual file system API.

refer to open and save. The logic is really straightforward:

for open:

• Get the file path and import the data
• trans the data to flowgraph
• update flow graph

for save or save as

• Get the saved name
• export the data to the YAML file under the specific path name

I failed with the directly logging to ensure whether they are called correctly to get the file path/name, which means the official QFileDialog.getOpenFileName does not work well. (Sad)

for copy and paste:

Since the upstream qt_gui in gnuradio has not implemented the copy and paste full feature, I try to call the origin flowgraph::copy/paste related method, with the flowgraph update. But seems there lack of some block connections with the transplant of the copy && paste.

for new:

it will new flow graph and set the newer one to central.