mainview: support close to systray

- Introduces a C++/Qml common mechanic for accessing system settings
- Refactors a good chunk of application wide settings management code
- Refactors the onboarding and quit-suppression logic
- Makes a first stab at cleaning the Qml object registration
- Removes some 'ClientWrapper' use
- Fixes some file name spelling errors and inconsistencies

Gitlab: #51
Gitlab: #53
Change-Id: I3dd1085aca72a38827ec004e347bd91106be2bcb