fix(windows): Warn if we reach maximum transient languages

Fixes #3749.
Fixes #3759.

Adds a warning dialog when user attempts to add a transient language to
a keyboard but the maximum number of transient languages is already
installed.

If this issue arises when a user is installing a keyboard for the first
time, Keyman instead installs the keyboard for the user's default
language. This may be a little confusing, but the error condition is
difficult to explain and non-technical users will probably be stuck
and need to install under their default language in any case. Happy to
receive any pushback on this decision.