Add Qt5 support

Adds Qt5 support to QGIS.

To compile
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Make sure you have the dependencies compiled and linked agains Qt5 also.
Most notably:

 * QWT
 * QWTPolar
 * QScintilla2

Start with a clean build directory

    cmake -DENABLE_QT5=TRUE -DWITH_BINDINGS=FALSE -DWITH_INTERNAL_QWTPOLAR=FALSE ../QGIS

You may have to adapt the paths to manually built Qt5 dependencies (as listed above)

    make

TODO
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Required
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 * ~~If ENABLE_QT5 is TRUE, Qt5 will be a hard requirement~~

Optional
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 * Evis plugin is currently using `QHttp`. This class is no longer available in Qt5. Port this code to `QNetworkAccessManager`. Currently it's disabled for Qt5 builds.
 * Introduce black magic in CMake that finds PyQt5
 * Find a solution for QWTPolar that works for debian based distros. Possibilities: 
   * Verify internal qwtpolar compatibility with Qt5 and update if appropriate
   * Build external packages for upstream (personal favorite)
   * Disable support for QWTPolar based features when external QWTPolar is unavailable