Add `-Wmissing-signatures` to `ghc-options`.

Add `-Wincomplete-patterns` to the `ghc-options` field of the cabal file
and fix all warnings.

Remove help check from parseFlags (redundant), add version check in parseFlags, add CPP flag to access VERSION_python_kernel in Flags.hs

+ Some minor formatting fixes.
+ Add IHaskell.Display.Widgets.Types to hindent ignored_files.
+ Remove some extensions and unused imports.

- Return of the newlines: Add all newlines once again
- Provide better comments
- Remove `sender`
- Consistently use set and get prefixed functions