Added window methods to request and to check focus

Signed-off-by: Stefan Schindler <stefan@boxbox.org>
Signed-off-by: Jan Haller <bromeon@gmail.com>
Added OS X impl of requestFocus and hasFocus

Signed-off-by: Foaly <foaly.f@web.de>
Signed-off-by: Jan Haller <bromeon@gmail.com>
Slightly adapted window focus documentation
Changed Window::requestFocus() return type from bool to void

Reasons:
* Consistent with other sf::Window methods
* User can test whether focus succeeded by subsequent hasFocus() call
* Implementation would have to call hasFocus() anyway on some systems

Also: minor code style change in Window::hasFocus()
Window focus: Added empty implementations for Android and iOS
Window focus on X11: check if window is viewable
X11: Notify instead of force focus

Signed-off-by: Jan Haller <bromeon@gmail.com>
Implemented case differentiation for window focus/notification on Windows and X11
Improve flexibility of dependencies locations on OS X

 - Replaced @executable_path by @rpath for more flexibility
 - Updated freetype and sndfile libs as follow:

    install_name_tool -id "@rpath/../Frameworks/freetype.framework/Versions/A/freetype" freetype
    install_name_tool -id "@rpath/../Frameworks/sndfile.framework/Versions/A/sndfile" sndfile
Updated Xcode templates

Since the @loader_path was replaced with @rpath, to correctly load
binaries from within a bundle app we need to add the Frameworks
subdirectory to the run path list.
Fixed documentation warnings

 - WindowImpl.hpp:94:16: warning: parameter 'threshold:' not found in the function declaration
 - SFContext.hpp:120:16: warning: parameter 'enabled:' not found in the function declaration
Merge branch 'feature/rpath_osx'
Merge branch 'feature/window_focus'
Added missing WINVER to fix a compilation error with the TDM-2 compiler.
Added .gitattributes file for proper line endings.
Minor documentation fixes in sf::String