Added support for blank placeholder spaces.

[xrolfex]

This PR adds support for a new object type of placeholder. It offers a way to put in blank spaces to allow for custom layouts.

This is one way to address #313. It still needs localization and is a first attempt. Im not sure if this is how you would like to handle it.

[matt8707]

Looks good. It's somewhat related to sidebar > divider > empty option. So, it would be nice to maintain consistency in appearance. However, this could lead to confusion with "new object". So maybe that style but gray in color, what do you think?

[xrolfex]

So something more like this? Are you also thinking remove the placeholder text as well?

[matt8707]

Yeah I'm thinking,

• Make it transparent white to differentiate from the yellow
• Use $lang('empty') instead of 'placeholder' (unsure if needed)
• Internally, rename it to 'empty'

[xrolfex]

Yeah I'm thinking,

• Make it transparent white to differentiate from the yellow
• Use $lang('empty') instead of 'placeholder' (unsure if needed)
• Internally, rename it to 'empty'

Got these taken card of; did remove the text during editing:

[xrolfex]

Requested changes applied.