Renames DolphinViewContainer::getCaption to caption

Summary:
This commit renames the DolphinViewContainer::getCaption to simply
caption, this was a review comment that I failed to fix in a previous
commit.

Test Plan: None

Reviewers: elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D15114