Fetch dataset information using web socket

[BECATRUE]

Now, the dataviewer fetches dataset through web socket implemented in snu-quiqcl/artiq-proxy#117.

In GUI, the only modification is that the button to update dataset list is removed.

I tested that

• the dataset list is updated in real-time.
• the selected dataset is fetched in real-time.
• clicking the stop button is reflected immediately.

For test, the following example experiment may be helpful.

import time
from artiq.experiment import *

class DatasetTest(EnvExperiment):
    """Dataset Test"""

    def build(self):
        self.setattr_device("core")

    @kernel
    def run(self):
        self.set_dataset("test_a", [], broadcast=True)
        self.set_dataset("test_a.parameters", ["shot"], broadcast=True)
        self.set_dataset("test_a.units", [""], broadcast=True)
        time.sleep(5)

        for cnt in range(10):
            self.append_to_dataset("test_a", [cnt, cnt])
            time.sleep(1)
        time.sleep(5)

        self.set_dataset("test_a", [], broadcast=True)
        time.sleep(1)
        for cnt in range(10):
            self.append_to_dataset("test_a", [cnt, cnt])
            time.sleep(1)

This closes #236.

[BECATRUE]

I realized that this app fetches the dataset list from the proxy server regardless of whether the current status is "realtime" or "remote". The correct implementation is that it fetches the list only when the current status is "realtime", but I will handle this in the next PR, #258, hence please never mind.