A few minor fixes I picked - mainly with rename action

* Fix the rename action to work on folders with `.` in them. E.g. before renaming `a.folder` to `a` (i.e. removing the apparent folder extension) would fail.
* Fix the rename action to allow changing file case (e.g. moving file `A.txt` to `a.txt`). OS X is case insensitive (so the move previously failed) but is case preserving (so we should respect the case)
* Show an error message when assigning a mnemonic doesn't actually work. E.g. assigning `abc` as a mnemonic to the file `/hij.txt` (i.e. there are no matches). Other ideas: we could ask the user if they want to set up a synonym?
* In fixing above, pass the whole object to the assign abbrev method, and get the identifier *there* (so that we can do more useful things with the object)