Force keyboard focus on the TextEditor control in DecompilerTextView

[siegfriedpammer]

There is one issue that remains: if keyboard focus has never been on the ICSharpCode.AvalonEdit.TextEditor of a particular tab, then pressing Ctrl+Tab from the Assemblies Pad will not move keyboard focus to the TextEditor when Ctrl+Tab is released to move focus to that tab. If, however, one is able to get focus into the TextEditor in some other way so that arrowing around within the disassembly actually moves the caret, then after pressing F6 to get to the Assemblies Pad, Ctrl+Tab when release will put focus back inside the TextEdditor so that a screen reader user can use the arrow keys to move around and read the disassembly.

What is really needed is for keyboard focus to always be on the TextEditor after using Ctrl+Tab to focus a disassembly document tab.

Originally posted by @glenrgordon in #2577 (comment)