fix(mac): support missing alt layers in OSK

[sgschantz]

When selecting an alt layer (corresponds to the option key on Mac) of the on-screen keyboard, display the keys that are output from Keyman for the designated layer as defined in the keyboard's KVK file.

Fixes: #5721

User Testing

• TEST_RALT_KEYBOARD_WITH_OSK: Khmer Angkor uses only RALT rules

1. Install this version of Keyman and switch to the Khmer Angkor keyboard
2. Open the OSK
3. Click on the Alt key on the OSK to display its Alt layer
4. Confirm that the L key displays the character '៘'

• TEST_ALT_KEYBOARD_WITH_OSK: Tibetan EWTS uses only ALT rules

1. Install this version of Keyman and switch to the Tibetan EWTS keyboard
2. Open the OSK
3. Click on the Alt key on the OSK to display its Alt layer
4. Confirm that the 7 key displays the character '༧'

[keymanapp-test-bot]

User Test Results

Test specification and instructions

• ✅ TEST_RALT_KEYBOARD_WITH_OSK (PASSED) (notes)
• ✅ TEST_ALT_KEYBOARD_WITH_OSK (PASSED) (notes)

Test Artifacts

• macOS
  • Keyman for macOS

[dinakaranr]

Test Results

Installed "Keyman 18.0.126-alpha-local" build on the macOS Sonoma 14.5. Installed the Khmer Angkor and Tibetan EWTS keyboard.

• TEST_RALT_KEYBOARD_WITH_OSK (Passed):

1. Select the Khmer Angkor keyboard
2. Open the OSK
3. Open the Notes app
4. Click on the Alt key on the OSK to display its Alt layer
5. Verified that the L key displays the character '៘'
   It works well, There is no problem when pressing the right Alt key on OSK.

• TEST_ALT_KEYBOARD_WITH_OSK (Passed):

1. Select the Tibetan EWTS keyboard.
2. Open the OSK
3. Open the Notes app
4. Click on the Alt key on the OSK to display its Alt layer
5. Verified that the 7 key displays the character '༧'
   It works well, There is no problem when pressing the right/left Alt key on OSK.

[keyman-server]

Changes in this pull request will be available for download in Keyman version 18.0.131-alpha