this meant that if some line were supposed to be indented in the third
column and you had two tabs and a space on that line, the indenter
would leave it alone. Now, if it sees tabs in the line anywhere, it just
decides that the line is not properly indented and re-indents it
(always using spaces)
Also: started a tabber test suite
then, use that to change how it works for the scheme mode (and also another variation for the REPL to
cope with the prompt)
I spent a while trying to make this work at the keymap% level (ie putting different keybindings for "home"
and "c:a" into different keymaps) but this just turned out to be far too confusing and fragile, so went
with this alternative (one keybinding, but that delegates to an overridable method)
closes PR 11446
