qt: Avoid HTML markup in accessible descriptions and names

* qt/pinentrydialog.cpp (PinEntryDialog::setFormattedPassphrase,
PinEntryDialog::setConstraintsOptions):
Set accessible descriptions and accessible names without HTML markup.

We use HTML markup for some tooltips and labels for nicer display. To
prevent screen readers from reading this HTML markup we need to set
the accessible description (takes precedent over the tooltip) and the
accessible name (takes precedent over label text) to texts without
HTML markup. (Note: For the label text Qt automatically tries to
remove HTML markup when asked for the description, but for consistency
and better control over the result we do it explicitly.)

• GnuPG-bug-id: T5543