Add Reset Zoom Level action inside View menu

Summary:
FEATURE: 409591
FIXED-IN: 19.12.0

Test Plan: {F7096397}

Reviewers: elvisangelaccio, shubham, #dolphin

Reviewed By: elvisangelaccio, #dolphin

Subscribers: broulik, cfeck, kfm-devel, kde-doc-english

Tags: #dolphin, #documentation

Differential Revision: https://phabricator.kde.org/D22444