Replace QObject wrapping boilerplate with lambdas

The resulting code is now much shorter and IMHO also easier to read & reuse.
Split Base page to Header page and BasePage

Header Page provides the content box, back button and header content box, BasePage adds a configurable header text. The main motivation fort his was to enable placing custom elements to the page header. Such as a search filed, etc.
Add attribution for the singpost icon
Fix rendering issues with background rectangle border
Use correct properties
Address search progress tracking
Add search menu page
Encode string with utf-8

This should get rid of some Unicode strings causing tracebacks
Add address search page
Add signpost and Wikipedia icons

The singpost icon is used for address search, the Wikipedia icon will be used for Wikipedia search.