Fix sidebar search field with fractional scale factors

Summary: By using a QQC2 TextField, we improve the appearance when using a fractional scale factor without regressing anything for integer scale factors and no scale factor.

Test Plan:
Search still works, clear button still works, appearance is improved with a fractional scale factor. No change with an integer scale factor or no scale factor.

1.3x scale, before:
{F6124306}

1.3x scale, after:
{F6124307}

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14256