a11y: Qt Creator's editor is no longer screen reader accessible

Unlike with earlier versions of Qt Creator, the editor is no longer screen reader accessible in the current development version of Qt Creator.

Steps to reproduce:

1) start the Orca screen reader

2) start Qt Creator, open any file for editing

3) move the cursor around in the file using the arrow keys

Actual result:

Orca doesn't announce the text to which the cursor moved (e.g. the line when moving to a different line using the up/down arrow key).

Expected result:

Orca should announce the text to which the cursor moved.

Further information:

1) This is a regression. Doing a local revert of

commit dd9681b0e9b5366c2e785d25bd3953c681849621
Author: David Schulz
Date:   Wed Feb 26 13:01:29 2025 +0100

   Editor: use forked plaintextedit

makes the announcement work again.

I suspect that if Qt Creator wants to use/maintain its own fork of QPlainTextEdit, it would also have to copy over and maintain the corresponding accessibility implementations from qtbase, too.

2) Crashes due to QTBUG-136074 may be encountered while testing, but that is a separate issue that most likely needs to be fixed inside qtbase.