Support undo and redo in input fields

How to reproduce the bug:
1. Open a text file;
2. Edit something in the file;
3. Use Cmd+F to activate the search box on the top right;
4. Type a keyword you want to search in the box;
5. Change the keyword in the box; (e.g., add a character to the keyword)
5. Use Cmd+Z to undo;
6. You will notice that Visual Studio Code actually undos the text file edit, not the search keyword edit.
