platform detection: qt5: Use the QPA platform name instead of the removed Q_WS_* #defines
platform detection: Remove support for Symbian

This is dead code anyway, Trojita has never run there, AFAIK, and the platform
is now EOLed. When someone does the port, they can easily dig this from the
history.
platform detection: Recognize MacOSX 10.8

The docs for Qt 4.8 say that that the enum value is available in there; let's
trust that.
platform detection: Add support for Windows 8

The documentation says that this is *not* available on Qt 4.8, so it's a
Qt5-only feature for now.
Improve platform detection -- adding new platforms and support for Qt5

REVIEW: 116564