Conan: Support using cmake, qmake via virtualenv generators

We aim to support building Qt examples that depend on Conan modules from command line, with minimal setup required by the user. The virtual[run]env generator seems to be ideal for this:

export QTDIR=<qt_dir>
conan install --generator virtualenv <src_dir>
activate.sh # Sets up CMAKE_PREFIX_PATH etc
cmake -D CMAKE_PREFIX_PATH=$QTDIR <src_dir>
cmake --build .
./example

export QTDIR=<qt_dir>
conan install --generator virtualenv <src_dir>
activate.sh # Sets up QMAKE_PATH
$QTDIR/qmake <src_dir>
make
./example

Extend the module recipes so that following environment variables are set:

QMAKEPATH (root of the module's mkspecs directory)
CMAKE_PREFIX_PATH (root of the module's lib directory)
QT_ADDITIONAL_PACKAGES_PREFIX_PATH (root of the module's lib directory)

and, on Windows,

PATH (pointing to module's bin folder)

Document for the examples in the respective modules