Description:

Please find attached a patch that proposes the following minor changes
to tab-line.  I can refactor the patch if we decide that some change are
not worth or if it's better to split it into smaller patches.

1) Turn the following defvar's into defcustom's:

- `tab-line-tabs-buffer-group-function'
- `tab-line-tabs-buffer-group-sort-function'
- `tab-line-tabs-buffer-groups-sort-function'

2) Add conditional around function call of
`tab-line-tabs-buffer-groups-sort-function', in
`tab-line-tabs-buffer-groups'.  This guards against nil values, similar
to how `tab-line-tabs-buffer-group-sort-function is handled, making the
behavior more consistent across these two (similar) variables.

Should we announce these "new" defcustom's in NEWS?