Clear button and busy indicator for search field

- add a clear button
- add a busy indicator
- restrict width to a maximum width to avoid that it spans the full
screen in landscape mode or on large screens (e.g. tablets)

The buttons (including the existing search button) are rendered inside
the search input field for a nicer look. The busy indicator uses a
custom style because the default Qt implementation fails to adjust its
size on Android.

REVIEW: 124605