New help images for FormPull, FormRebase, FormMergeBranch

This pull request contains:
1. Add user control to display up to two help images
2. Improve resource image file naming and prepare for different images in Pull, Merge and Rebase dialog
3. Use the new user control in FormPull, FormRebase, FormMergeBranch
4. Use new help images in FormPull, FormRebase, FormMergeBranch
5. Fix some minor form issues
(screenshots follow)