Make the search box more compact

Summary:
Reduce the spacing between widgets and remove the content margins on the extended search options to have a more compact and space-saving layout.
Currently, some widgets are not even aligned.
This will be even more important if we remove the toggle button for 'More Options'.

CCBUG: 386754

Test Plan:
BEFORE: {F7681182}
AFTER: {F7681184}

Reviewers: elvisangelaccio, ngraham, #dolphin, #vdg

Reviewed By: elvisangelaccio, ngraham, #dolphin, #vdg

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D25028