libcore|libgui: Added more Core.String bindings, gui.dpiScaledImagePath()

Makes it easier to use "@2x" images.
Default Style|Client: Use gui.dpiScaledImagePath()
libappfw|VRConfig: Determining if VR mode displaces content on screen
qmake|Builder: Building and deploying packages

Added a new Python script for building a '.pack' package. Added
qmake macros for building and deploying packages.
libcore|libgui|qmake: Building and deploying packages

The stdlib and stdlib.gui packages are no longer built by packres.py.
The "buildPackage" macro is used to build them, and deployLibrary()
will automatically deploy the packages if needed (for SDK).
Tests: Added a test application for libappfw

See the README.md for details.
libcore|App: Added utility for finding files from loaded packages
Fixed|SDK|X11: Various fixes for Qt 5 and X11 specific code

These changes allow test_appfw to run on Linux (with -basedir pointed to
the right directory).
qmake: Deploying built packages

Packages built with buildPackage() are now properly installed when running
"make install" in a non-SDK build.
qmake|Resources: Continued cleaning up package building

The 'client' and 'base' packs are now created using buildpackage.py
in the appropriate .pro files.