Remove foreach in KmPlot in favor of range-based loops

Summary: foreach is obsolete

Test Plan:

1. Compile, install, run.
2. Create some plot (sin(x)), save it, export it, close it, load it.
3. Use equation editor.

Reviewers: KDE Edu, aacid

Reviewed By: aacid

Subscribers: aacid, kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D24212