accountcombobox: cleanup and simplify qml layer

- remove some faulty hover state management, excessive text metrics
  objects. Split the code up too and increase declarativeness.
- fix the rendering of account avatars in the dropdown list
- make the 'add account' item always visible
- make the popup modal with transparent overlay

Gitlab: #388
Change-Id: If0e5f95d1250ee77902c518469878bdb678c8b9d