Allow more control over invocation of completion models when automatic invocation is not used.

• Starting several models at once
• Using an invocation type other that ManualInvocation (important for CodeCompletionModel::MatchReaction::HideListIfAutomaticInvocation)
• Clean way to retrieve list of all registered models (including the KateKeywordCompletionModel)

Combined, these additions allow to inplement a complete custom version of KateCompletionWidget::automaticInvocation().

Provided as CodeCompletionInterfaceV2 (to be merged into CodeCompletionInterface for KF6) as suggested by @cullmann in !3 (closed) .