Do not use dropdown-list + change keybindings + improve checking with typed holes

This pull request has three different sets of changes:
* Moves the interface for selecting a value for automatic completion from using `dropdown-list` to the initial implementation, improved with automatic focus and killing when the selection is done, as required in #312. This eliminates the dependency on `dropdown-list` package, fixing #318.
* Changes the keybindings of case splitting and code generation to `M-t` (integrated with the old code) and `C-u M-t` respectively, as said in issue #316.
* The checking process now runs twice on each file, with and without typed holes support, to get all the error messages to the user, and fixing #310. The implementation is better than just deferring type errors, because it  does not need a list of all possible type errors, which may grow in future GHC releases.