#15033 Editor: preserve selected text (if any) when inserting a file.

[dannylagrouw]

Fix for #15033 (on inserting a non-image file, any selected text in the editor is overwritten with the file's title). This fix will preserve the selected text and link the file to that. If no text is selected, the file title is inserted. However...

• I'm not sure if I used the right way to get to the selected text (this.tinymce.activeEditor.selection.getContent), I didn't see another easy way.
• If you insert multiple files at once, the selected text will be repeated for every inserted file. I'm not sure if that's what you'd expect.
• I got a lot of lint warnings on vars in the existing source file (markup.js). I replaced these with consts to be able to commit.

[matticbot]

@dannylagrouw This PR needs a rebase

[apeatling]

Closing this because it is old and inactive. @aduth if you're willing to give this a review please reopen.