GUI: don't forget to set proper text color in non-default backgrounds

Thanks to "serpentine" on IRC for this feedback.
qt5: fix build failure with Qt 5.0.1

Got to fix for the qtbase's commit 1342f5da382532d75b381e0dd303cb57827a0d29.
Added a QAction to View Message Source.

Wrote a new class "MessageSourceWidget" that is responsible for viewing
the message source.
Merge remote-tracking branch 'ahmed/view-message-body-header'
Fix Ahmed's name
Make the source viewer bypass the EmbeddedWebView entirely

It isn't clear to me why using it through the SimplePartWidget works just fine,
but going through the EmbeddedWebView hides the scrollbars upon loading text
which is already in the cache. Weird.
Coding style: whitespace fixes, QLatin1String stuff