Use system fonts in webviews: Bug 713746

Set the WebSettings default and monospace fonts from the system
settings.  Also, use "font: caption" to get the UI font for UI elements
in the webview.  The former need to be updated each time they change.
The latter is automatically updated, though that only works
sporadically.

Unfortunately, this leaves messages in the conversation view in the UI
font, not the document font, because webkit treats "font-family:
initial" like "font-family: inherit" for some reason.
Set document font explicitly with CSS in conversation viewer: Bug 713746

This avoids the previously-mentioned bug in "font-family: initial".

Note that if you set the font size to 11pt in CSS, for example, you get
15px.  But if you set it to 11pt with the WebSettings and then set that
size with font-size: medium, you get 14 px.