Discussion: bufferline/statusline/picker elements

[sbromberger]

Now that #2759 has been merged, it might make sense to take a larger look at how we provide consistent display of elements for bufferline, statusline, and the various pickers. For instance, the file modification indicator is hard-coded in the various "render filename" routines used by all three. I think for consistency we should do something like the following:

1. create a file-modification-indicator (similar to the read-only-indicator proposed in read only indicator #3709) element that can be placed in statusline
2. allow bufferline to have a specific configuration like statusline does
3. allow pickers to have a specific configuration.

Alternately, we can have a common render_filename function that will add these things statically, without customization capability.