Fix bug parity not closed

While playing with Electron installed via .dmg, I found out when parity is launched by UI, and when I close UI, parity actually doesn't stop (different behavior when using npm run electron). This PR forces parity to stop when electron stops.

Also rename Parity Wallet -> Parity UI.

@jacogr If you want to test this, in https://github.com/Parity-JS/shell/releases there's a draft version which contains binaries with this PR's changes. I tested on windows and osx and both works, would like you to play a bit with it too.