The cleartext button in the passwordbox doesn't match the one in a textbox

[henrik-oscarsson]

There were some triggers missing. See the screenshot below that illustrates how the cleartext button looks when hovering the mouse over it (on a textbox):

Also, it changes color when pressed (as expected).

Here's a screenshot of the password box, when hovering the button:

You can see that the 'X' becomes black, but the background isn't affected. This PR fixes that issue.

[henrik-oscarsson]

Fixed the foreground of the watermark text also, now it reflects the foreground set on the textbox/passbox "parent".
See screenshots below, the first one illustrates the (metro demo) passwordbox, having the foreground set to white, but this isn't reflected in the watermark text, it is still black:

After the bug fix, it looks like this:

The PR handles both textboxes and passwordboxes.