Fix Safari baseline bug for entries fields

[JulianRoesner]

Short Description

The delete entry button was not aligned in Safari (s. this ticket). In Safari align-items uses bottom border from empty input fields. Using an empty placeholder text as discussed here fixes this issue.

Changes

• Add a new Mixin, which can be used to set the placeholder text in widgets.
• Set the placeholder text also in the jquery_entries.html because the entries fields are set there

Feedback

• Do you have any better idea how to set the placeholder for the entries field than the added macro?

[frederike-ramin]

I like the macro fix, actually. Makes the entries field a lot cleaner imo.
Just one thing: I'm not sure SafariBaselineBugFixMixin is the right name as Safari as well as Firefox have this bug. So I'd probably just go for BaselineBugFixMixin instead.