gui: Move static placeholder texts to forms

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.

It's better to keep this kind of text content together. Makes sure
translate/no-translate status is kept as it is.

src/qt/sendcoinsentry.cpp

@@ -37,7 +37,6 @@ SendCoinsEntry::SendCoinsEntry(const PlatformStyle *_platformStyle, QWidget *par
 
     if (platformStyle->getUseExtraSpacing())
         ui->payToLayout->setSpacing(4);
-    ui->addAsLabel->setPlaceholderText(tr("Enter a label for this address to add it to your address book"));
 
     // normal bitcoin address field
     GUIUtil::setupAddressWidget(ui->payTo, this);