Add context to tab labels of Content and Text widget editors

[pedromendonca]

Add context to improve consistency of the strings of Visual and Text (formerly HTML) tab names of the content editor and the text widget editor, keeping it separate from generic strings.

Because of its shorten names, for some languages these tab names translations might not be as simple and direct as it seems.

The tab name Text means Text mode, but it's not as clear as the old HTML, and just Text oposing Visual might lead to some confusion, as both can be about text.

But this ticket isn't about opening this discussion, and that was already decided previously in this ticket: #20993.

This ticket adds a context to these strings to allow the Translation Teams to choose the better translation for each, keeping the consistency between the tabs of the Text Widget editor and the Content Editor, without compromise the current translation of a generic string "Text" (or "Visual") that have the potencial to be used a lot within core and meta.
Eg. Text Widget title

Current Text Widget editor tabs:

__( 'Visual' ), // Editor switch tab label
__( 'Text' ), // Editor switch tab label

Current Content editor tabs:

__('Visual')
_x( 'Text', 'Name for the Text editor tab (formerly HTML)' )

Proposed Content/Text Widget editor tabs:

_x( 'Visual', 'Name for the Visual editor tab' )
_x( 'Text', 'Name for the Text editor tab (formerly HTML)' )

[pedromendonca]

Patch to add context to the editors tabs labels

[pedromendonca]

Small correction to contexts

[SergeyBiryukov]

In 41152:

I18N: Use a consistent context for Visual and Text editor tab labels.

Props pedromendonca.
Fixes #41438.