Add auto-complete feature to texteditor

[X39]

Currently, the texteditor features no autocompletion.
AvalonEdit (the actual editor control used) however, supports this easily.

Goal

• Enable Auto-Completion using AvalonEdit
• Implement a way for text-editor-providers to check auto-completion

Related

• https://github.com/ArmA-Studio/Arma.Studio/blob/master/ArmA.Studio.Data/UI/CodeEditorBaseDataContext.cs#L151-L169
• https://github.com/ArmA-Studio/Arma.Studio/blob/Rewrite/Arma.Studio/UI/TextEditorDataContext.cs
• http://avalonedit.net/documentation/html/47c58b63-f30c-4290-a2f2-881d21227446.htm