Add theming to hide text cursor if no text is put in

[J0hannes101]

This PR adds a theming option to hide the text cursor when the input is empty, as discussed in #2105.

It works as expected, but we still need to decide on:

• The naming of the theming option (currently hide-empty-cursor: true)
• Which widget should be used to place the hide-empty-cursor option in a theme (currently in entry{})

Also need to update the docs.

[DaveDavenport]

The name of the widget is the right place to place this option on.
Need to think about naming.. it now sounds like hiding an empty cursor, not hiding cursor on empty textbox.
Maybe 'hide-cursor-on-empty'?

[DaveDavenport]

Thanks.