Quicktag end tags appear after newline characters in text editor

[JoshuaGoodwin]

In order to select a whole line of text in a textarea, it's often convenient to use the triple-click shortcut. In WebKit/Blink browsers, if the selected line of text is not the last line, the selection includes the newline character at the end. So when I click an editor quicktag button, the closing tag appears on the next line.

This is perhaps best illustrated with a screencast: ​https://mediacru.sh/d6KyDiACDuri

Instead, perhaps the closing tag should be inserted before the newline character.

I've tested on using Chrome 37 on Windows and Linux, but it's a long-standing thing.

[JoshuaGoodwin]

A patch for qt.TagButton. wpLink and qt.insertContent() probably need changing too.

[JoshuaGoodwin]

A patch for qt.TagButton (with the correct index, sorry)

[iseulde]

Looks good! Thanks for the patch and apologies for the long wait.

[azaozz]

Thinking it's better to adjust endPos when the last char before it is \n. Pretty similar but perhaps a bit cleaner :)

[azaozz]

In 37661:

Editor quickTags: when the user selects some text by triple-clicking, then wraps it in a tag, and the last selected char is \n, insert the closing tag before the line break.

Props JoshuaGoodwin azaozz.
Fixes #29571.