fix(text area): end mouse selection only if currently selecting

[TomJGooding]

The TextArea should only invoke the _end_mouse_selection method if we're actually currently selecting with the mouse.

This will fix #4434 where currently the cursor blink is restarted after the TextArea is hidden in the TabbedContent.

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[darrenburns]

Thanks @TomJGooding - very minor, but would the if self._selecting maybe be better off inside the _end_mouse_selection if it's always required?

[TomJGooding]

@darrenburns No problem, that's now updated. I did wonder that myself but thought the other way might better express the intent.

[darrenburns]

@TomJGooding Thanks for making that change and for the fix!