bug(windows,linux,mac): Config should warn if package contains no Windows keyboards

**Describe the bug**

If a package contains no Windows keyboards, then the Install Keyboard interface should warn the user prominently if they attempt to install it.

**To Reproduce**

1. Select a keyboard which is js-only (e.g. [sliammon.zip](https://github.com/keymanapp/keyman/files/5705709/sliammon.zip))
2. Attempt to install
3. No warning is shown -- the keyboard appears to 'install'

**Expected behavior**

Either a warning message or prevent the user from installing. (There is a potential valid use case of installing a fonts or documentation-only package, or similar, that we may want to consider.)

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**Keyman for Windows/macOS/Linux:**
- OS: Windows 10 20H2
- Keyman version: 14.0.210-alpha