Add user control to display up to two help images.
Help images: improve resource image file naming and prepare for different images in Pull, Merge and Rebase dialog
Enable autosize for HelpImageDisplayUserControl
FormPull: use HelpImageUserControl instead of PictureBox
FormRebase: use HelpImageUserControl instead of PictureBox
FormMerge: use HelpImageUserControl instead of PictureBox
HelpImageUserControl: white background and autosize
FormPull: Use new help images
no autosize
FormMergeBrance: use new help image
FormRebase: use new help image
FormMergeBranch: sizing fixes
FormPull: show fast forward help image version on hover
FormMergeBranches: show fast forward scenario help image on hover
Help images: fix distances for hover
FormPull: add merge and rebase icon
Merge branch 'master' into topic_help_images_for_merge_and_rebase

Conflicts from 5368be37211c6b2cb269f0761e9bc7fa83251e85 :
	GitUI/FormMergeBranch.Designer.cs
Merge pull request #1505 from feinstaub/topic_help_images_for_merge_and_rebase

New help images for FormPull, FormRebase, FormMergeBranch