Cache the layout list in the service app

[TolikPylypchuk]

Currently when the service app switches text it asks the OS for the list of the layouts each and every time. This list is changed quite rarely, and even when it is, the new languages should be configured, so the service app should just cache it.

The service app should get the list of layouts once at startup and then update it only when the settings are reloaded. This means that the user must reload the settings even when they delete layouts, which wasn't the case previously.

[TolikPylypchuk]

On Windows the layouts are already cached, but this cache is never invalidated, which requires the user to reload the service app when they change the layouts.