gui: Move static placeholder texts to forms

Summary:

There was an issue around the time of Qt 4.6 when placeholder text was
introduced, that caused a compile failure when it was specified in the
form. As a workaround the placeholder texts were moved to the code.
Qt 4 hasn't been relevant to us for ages. So move all (non-parametrized)
placeholder texts to the form files instead.

The change to openuridialog.cpp/.ui is not included, because it is parametrized:
the placeholder text can be bitcoincash: or bchtest: or bchreg:

This is a backport of Core PR17702

Test Plan: ninja && src/qt/bitcoin-qt

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8304