MS Edge support

[cutright]

Although I've been able to get the MS Edge backend (rather than IE) to work in DVH Analytics, I'm having some issues here. IQDMA pretty much entirely locks up.

Work-around:
Don't use MS Edge. The downside is you don't get any drag functionality in the plot (e.g., pan/zoom).

I may look into CEF Python for chromium support instead: https://github.com/cztomczak/cefpython

[cutright]

After an exhaustive trial and error, the issue was that the wx.html2.webview could not live in the top-level wx.Panel.

Unfortunately, placing the webview in a child panel on macOS would cause the webview not to respond to mouse motion events unless the mouse was in a clicked state.

Work around is two separate layouts.

[cutright]

Will be available starting in v0.1.7 (and currently in the v0.1.7 branch)