Change Electron detection to be user agent based only

[Calyhre]

The Electron detection is currently base on the presence of window.process OR the user agent string.

This works fine for the mxIsElectron, but not for the Electron > 5 check as then we assume window.process is present. This is false for webviews and iframes. Draw.io is currently crashing if used inside an iframe, inside an Electron app.

This PR change the Electron check to be based solely on the user agent string.

[davidjgraph]

What about a defensive check on the electron 5 line for process? We've been bitten by useragent checks before...

[Calyhre]

I didn't dig this whole things up, but isn't the Electron check just to see if the app is loaded in drawio-desktop? If so, then maybe we should fail the check at all if window.process is absent. If the app is loaded in an iframe there is no need to know about it, as it won't be in the desktop app.

[Calyhre]

I think it is, later the imports and URL handling are changed depending of this detection. There is no point of doing this if loaded from an iframe. I changed the check to just detect the presence of process.

[davidjgraph]

OK, I see your point. I'm just thinking about an electron app with node integration enabled that loads draw.io in a web view. We've a lot of security controls based on the electron setting. I'll pull this change and leave this note here as a warning to future readers:

If you load draw.io in an iFrame or WebView (I think process isn't there when node is disabled), you need to look at the places where we tighten security based on this flag. If you have the node integration enabled, you must not load the draw.io site as is. Third-party JS is loaded for some of the cloud integrations and if they were compromised the app could gain access to the local filesystem.