[win32] Explicitly set Qt plugin search path

Qt5 on Windows requires a platform plugin/DLL named qwindows.dll. The default
search path seems to be hardcoded at compile-time to the location of the Qt
libraries on the machine where the code is compiled. It doesn't seem to be possible
to override the search path using qt.conf and Qt also doesn't search the working
directory of the binary. Thus we explicitly add "./qt_plugins" to the search paths
by calling QCoreApplication::addLibraryPath(). Then qwindows.dll has to reside in
"qt_plugins/platforms". SVG icons then work as well if qsvg.dll is available in
"qt_plugins/imageformats".
Related bug report in Qt's official bug tracker: https://bugreports.qt-project.org/browse/QTBUG-28766
[win32] Transition build system to Qt5
[win32] Install platform and imageformat plugins

Qt5 requires a platform plugin to work at all. Additionally, we install the SVG
imageformat plugin so icons show up in UMVE.