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:

1.3x scale, after:

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

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