Add GUI property input_init_value_destroyed_at_focus

[Langbart]

Currently the chat input field in the lobby has no default text e.g. Say something, which gets destroyed as soon as you select the field. The article ​wiki/GUI_-_Properties already lists this possibility, but says Not yet implemented (last edit 22/Oct/13).

It should look like in the image below, it comes from a game called: Wreckfest:

[Langbart]

The problem has already been addressed by Angen, just use:

• placeholder_text="Say something".

Edit: Updated the wiki article.
​https://trac.wildfiregames.com/wiki/GUI_-_Properties