Enable overstrike mode

[mgarstenauer]

This PR enables Overstrike mode (in other editors called OVR, Overwrite, Overtype), which is not yet working.

Simply uncommenting the code in TextArea.cs solves the problem. All relevant code paths for Overstrike from AvalonEdit are already present in AvaloniaEdit.

Instruction for testing:

• Run AvaloniaEdit.Demo.
• Press Insert (ins) key on the keyboard.

Expected behavior:

• The caret changes from a line to block.
• Typing now overwrites the right characters instead of inserting characters at the caret position.

[danipen]

LGTM