use standard way of adding widgets to status bar

Avoid too many lines around widgets in some styles.
set size of DocListButton to the size of buttons on toolbar

Get rid of magic constant 24.
don't set maximum sizes for elements in SearchPopup
don't set maximum size for threshold spinbox in AutocompleteSettingsPage
don't set maximum width for add/remove buttons

Also use icons instead of +/- texts if it's possible.
avoid setting maximum and minimum sizes for some widgets

Maximum and minimum sizes should be calculated based on
widget's content. In most cases Qt calculates reasonable
size hints by itself.