Improve Register page (#346)

* add username error label, add word wrapping for error labels

* make lint

* last commits

* remove font, and remove some lambdas

* add QRegularExpression for regex

* improve review things

* add isModified() for checkFields()

* add check isModified() for  all fields in checkFields

* Update src/RegisterPage.cpp

Co-authored-by: DeepBlueV7.X <nicolas.werner@hotmail.de>

* remove emit call for showError

Co-authored-by: DeepBlueV7.X <nicolas.werner@hotmail.de>