Initial suppport for multi-toolkit image support

Image objects loaded using a toolkit can now be used in
other toolkits.
Add public method for creating multi-resolution images
Improve support for cross-toolkit images

Allow specifying an image loader callback. Secondary toolkits will use it to
load images on demand.
Fix NRE
Merge branch 'xtookit-images'
[WPF] Track api changes

Image.ToImageDescription() now takes an ApplicationContext as argument,
so we need to pass it along everywhere that method is used.
Merge pull request #339 from sevoku/add-list-scroll-to-row

Add ScrollToRow method to ListView/ListBox
Merge remote-tracking branch 'upstream/master'
Track API changes