Use relative heights

[TheSharpieOne]

It would be very nice to have the height of the placeholder be based on the height of the text that would be there; inheriting from the parent.
This should probably be easily achieved using ems instead of px

This is useful for placeholding headers without having to manually set the height. (Note: manually setting the height on the default type ('text') doesn't affect each row, only the height of the area as a whole)

[FrancescoCioria]

Hi @TheSharpieOne I agree with you, it's a good idea!
I think we can change the defaults to the equivalent in em based on a standard font-size of 14px.
I'll open a PR shortly with these changes

EDIT: actually it may be correct to make this a breaking issue and use 1em to have exact same height of the text