Provide a pure transparent TextBox theme.

[rabbitism]

This is originally implemented inside CalendarDatePicker. it turns out it is quite useful in other similar controls.

Notes:

• [x] Remove background/borderbrush and styles
• [x] Remove support for clearButton and password reveal button
• [x] Remove data validation
• [x] Keeps inner left/right content placeholder
• [x] Keeps watermark
• [x] Remove default Context flyout.

[rabbitism]

I will implement this first in Ursa, and later transfer back to Semi.