[HxInputBase] Application-wide default for LabelType

[hakenr]

Extracted from #872 for @mmonteagudo, where there were two features combined into single issue.

For now, I've decided not to accept the idea of adding the ValidationFor, ValidationForFieldName, and ValidationEditContext parameters to HxInputBase (i.e., to every input component).

However, I do like the idea of having a central place to set the default LabelType for all input components, and I'll try to finish that based on PR #873 (I'll revert the validation-related changes and fine-tune the LabelType idea).