Use consistent icons & text for the "Close" actions

Summary:
Consistently use "document-close" icon for close actions.

Actions on view containers were talking about closing "Files",
while actually they only closed the view (which would trigger
closing of the document/file only if this was the last view).
Talking just about "Close" without a target, like in the other
actions, avoids trying to explain this concept, average user
surely expects this, being a common pattern.

Also moving the close actions in the document toolview context menu
at the end, where close actions are usually placed.

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D6819