Code browser tool: add lock action text for toolview menu, use KToggleAction

Summary:
The toolview context menu for the code browser currently shows two items
with no text, as the actions are added only with a tooltip.
This patch fixes this for the Lock Current View action.

Target is 5.1, only new strings are added.

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4824