lsp: Almost inline rename UI

I'd like to have a 'rename' UI (for LSP/jedi), that:

Is in a virtual lines block below the identifier I'm renaming. Until this is stable, fallback to a floating window impl? (but it'll hide some text below..)
The UI should look like:
```
do_something_useful(some_variable)  # I want to rename `some_variable`
          Rename: new_variable|               # this line is a virtual line (and colored)
   Press <cr> to validate, <esc> to cancel    # and this line
```
- The editable area is aligned with the original identifier I'm renaming (note: there are special cases when the identifier is on the left/right edge of the window).
- The 2 lines of the rename UI are colored (darker than Normal? lighter?), and the Rename: text and the help text should be dimmed, to not be a
Highlight all visible identifiers that are going to be renamed.
Mappings: insert mode, with: <cr> to validate, <esc> to cancel (and tiny help line below to explicit that).
Has good support for completions (omni complete, keyword, auto complete, ..).
(If possible?) show the count of (visible/total) renames that are going to happen (and in how many files?). That information may be slow to get? try to do it async?
Ctrl-u should work properly (without deleting more than just the thing I'm renaming.

bew / dotfiles