Support reloading plugins

[JulienCochuyt]

WebAccess currently has a few issues when reloading plugins using NVDA+control+F3:

1. It does not attempt to remove its monkey-patching.
2. In some cases, its dialog may remain open in an unstable state.
3. Documents already opened have to be refreshed as we overlay on TreeInterceptor assignment.

This is no big deal and we usually simply restart NVDA during development phases, but it might be annoying for someone developing another addon while using WebAccess as a regular tool.