Change default web behaviors that don't make sense in the context of Electron

There are a number of behaviors that Electron inherits from Chromium which don't make sense for the vast majority of Electron apps. For instance:

- dragging a file into a window navigates to that file
- pinch-zooming in a window zooms the UI
- Cmd+Click opens links a new BrowserWindow

Electron apps that don't want these behaviors currently have to disable them explicitly, in an ad-hoc and error-prone way. Since these behaviors are undesirable for almost every Electron app, we should disable them by default.

This issue is a tracking issue for discussion about this general idea; separate issues will be created for specific changes that need to be made.