Clarify in the docs about what an invalid selection means, contrasted with a 0,0 selection.

Before this change, it was possible to select and copy obscured text from a text field.

This PR changes things so that:

- Obscured text fields don't allow copy or cut.
- If a field is both obscured and read-only, then selection is disabled as well (if you can't modify it, and can't copy it, there's no point in selecting it).

Updates the platform shims in dev/manual_tests so that Windows and Linux can be built. I had to update the Windows shims, because I was unable to build a Windows app there.

Also updates the analyze.dart script to report all license issues simultaneously instead of just dying after the first failure.

The only substantive code changes are in dev/bots/analyze.dart and dev/bots/test/analyze_test.dart

New parameter for InteractiveViewer: scaleFactor, which allows control over the speed of zooming.