Refactor for loops

Summary: For loops have been refactored to use more idiomatic JavaScript functions (filter(), some(), forEach()) where appropriate.

Test Plan: Ensure no regressions in modified components

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: broulik, ngraham, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D28666