Fix many problems in the completion box of the searchbar

* clearSuggestions was crashing to due unexpected reentrancy
(takeItem -> currentTextChanged -> textEdited -> clearSuggestions)
()

* completion box would pop up -after- hitting return, so it would eat
the first click in the webpage
()

* pressing the mouse on a suggestion item would also lead to a text change which
would clear suggestions, hard to use; solved by not doing this while the mouse
button is down. This problem still kind of happens with key down/up, but isn't
solvable under the current KLineEdit logic of "choosing text in completion popup
is equivalent to typing it".

(cherry picked from commit 713462a0cda593dd2c85bb0c221aba332f84edff)