Text Editor Placeholder

[MinusGix]

Improves #341

This PR does two things

• makes phantom text aware of which editor it is in, which was desirable anyway
  • Removes EditorTextProv as if it needs to the full Editor to pass to phantom text functions then it doesn't need to be a separate structure
  • Makes TextLayoutProvider::text return Rope rather than &Rope which I'd been thinking about anyway as Rope is cheaply cloneable
• Adds placeholder support to the inbuilt TextEditor widget
  • this uses the Dim color by, should it use a new/different color? It does look a bit too dark on the dark theme

(I implemented this on TextDocument (so it can be used via TextEditor easily) rather than making it a general Editor feature as Editor provides the general tool of phantom text to make placeholder easy, which keeps the Editor simpler.)

Of course this also gets us closer to being able to swap out the text_input so they use the same code.