krunner: Add indicator/toggle for singleRunnerMode

This mode was recently exposed as a command-line option to KRunner. The selected runner's name is part of the placeholder text, but when typing a query, you can no longer see it. Also, the only way to quit this mode is to reopen KRunner.

Instead, there is a simple button with the runner's icon indicating that we only query this one runner. When it is pressed, we switch back to normal mode and hide the button.

Depends on frameworks/krunner!146 (merged) and the milou counterpart.