Fixed TextBox watermark alignment

Fixes #603

MetroTextBox and SearchMetroTextBox styles:
1. Added template binding for watermark horizontal alignment.
2. Changed watermark margins for better look when vertical alignment
active.

MetroDemo:
1. Added some TextBoxes and ToolTips to show horizontal alignments.
2. Removed unnecessary vertical alignments.
3. Make equal appearance for headers in text tab (capital B in TextBox,
PasswordBox as was in RichTextBox).