feat(mac): user-controllable legacy app list

Fixes #1953.

Makes it possible to specify additional apps that need 'legacy' handling
without requiring an updated build of Keyman for Mac. The list should be
stored in user defaults under the `KMLegacyApps` key. It can be viewed
with the command:

```
defaults read keyman.inputmethod.Keyman KMLegacyApps
```

And updated with

```
defaults write keyman.inputmethod.Keyman KMLegacyApps -array value1 value2 ...
defaults write keyman.inputmethod.Keyman KMLegacyApps -array-add value1 value2 ...
```

Each entry should be a regex that matches the client app id.

For example:

```
defaults write keyman.inputmethod.Keyman KMLegacyApps -array com.microsoft.Word '^com.github.atom$'
```

(Note for purity, the `.` should be escaped in the regex but it'll
actually be fine without...)

This commit also adds the `keymanDataPath` method on
`KMInputMethodAppDelegate`, so we can place additional configuration
data there in the future (as opposed to in the shared config which is
less easy for users to edit.)