Rename Application Menu to Global Menu to avoid conflict

Summary:
We somehow ended up with two applets named Application Menu; kicker and
the global menu bar. Both having the same name is confusing.

This is a translation break on a stable branch, but it's a bug fix.

CCMAIL:kde-i18n-doc@kde.org
BUG: 375478

Reviewers: #plasma, mart

Reviewed By: mart

Subscribers: mart, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4454