[bugfix] Clicking 'Code Now' will open multiple files

[jdneo]

Fix #217

Bug repro step:

1. Click 2-3 problem node to preview the description one by one
2. Click Code Now
3. It will generate all the files for the problems that have been previewed before

Root cause:
The listeners are registered every time executing preview (). They should be registered only once when this.panel has not been created.

[Vigilans]

I tested the bug on my side:
Click problem [3], [70], [169], and the behavior appears as expected. Besides, after testing the behavior multiple times, one of the problem may fail to be shown for JSON parse error on input. After this PR, this problem may disappear.

Other flaws of the preview command:

• the extension will still fetch problem description after clicking the active problem node
• did not make use of local cache

[Vigilans]

The preview's code now works properly on my side now.

[jdneo]

Merged for now. We can improve both the preview and the result webview in 0.13.2 using the markdown solution, which I think can improve the experience significantly. Meanwhile, more bugs and some refactoring work can be addressed in 0.13.2.