entry: Use a label as placeholder

entry: Use a label as placeholder

This gives us a better way of choosing the color of the placeholder text
(and enabled general css styling on it of course).

Closes #378 (If you want to keep the placeholder on focused and empty
entries, just don't set the placeholder opacity to 0 in
entry:focus>placeholder. This is the default behavior but this commit
includes a rule in Adwaita to hide it.