Disable channel icon scan when icon path is empty

XBMC checks the icon path and terminates the scan early if no path is set. This commit causes the setting to respect that.
fix compile warning (assignment inside if)
fix HasExtension didn't necessarily terminate correctly
[osx] - fix windowed/fullscreen transitions with multi screen setups
[xcode] - remove stale references to ios6.1 sdk
Merge pull request #3516 from Memphiz/osxmultiscreen

[osx] - fix windowed/fullscreen transitions with multi screen setups
Merge pull request #3392 from garbear/pvrmenu.searchicons

Disable channel icon scan when icon path is empty
Merge pull request #3506 from jmarshallnz/compile_warnings

Compile warnings
CGUIMediaWindow: add UpdateFilterPath() method extracted from Update()
fix CGUIMediaWindow::OnPrepareFileItems overrides
add IFileItemListModifier interface and CSmartPlaylistFileItemListModifier implementation
add CFileItemListModification to handle all IFileItemListModifier implementations
CGUIMediaWindow: make use of CFileItemListModification
[win32] update VS project files
[osx/ios/atv2] - sync xcode project