Support multiple i18n branches with the same scripty
Adapt the behavior based on the new SCRIPTY_I18N_BRANCH variable. If the variable is unset, the default behavior is unchanged (i.e. tracking the trunk_kf5 branch).
The value of SCRIPTY_I18N_BRANCH should end by the major Qt version number. This has been the case so far, and as we control it, we can keep enforcing this rule.
Add a few basic definitions for trunk_kf6, in order to test it and bootstrap that branch.
Closes #4 (closed).