FormBrowse: Diff Tab: new context menu item: "Show in File tree"

- add new icon for IconFileTree to distinguish it from IconBrowseFileExplorer:
![2013-05-19_18-05-54_](https://f.cloud.github.com/assets/368412/523199/0e5c0d86-c09e-11e2-8dc5-73a09f59f5dc.jpg)

- disable context menu items for multiselection where multiselection is not handeled:
![2013-05-19_18-08-58_gitextensions feature_diff_goto_filetree - git extensions -_ debug _-](https://f.cloud.github.com/assets/368412/523206/84fce0f0-c09e-11e2-9ee7-2ef1930d72af.jpg)

- add context menu item "Show in File tree":
1: context menu:
![2013-05-19_18-04-12_gitextensions feature_diff_goto_filetree - git extensions -_ debug _-](https://f.cloud.github.com/assets/368412/523200/303e09fe-c09e-11e2-927d-f72e09b7e444.jpg)
2: result after click:
![2013-05-19_18-07-30_gitextensions feature_diff_goto_filetree - git extensions -_ debug _-](https://f.cloud.github.com/assets/368412/523203/41baae26-c09e-11e2-8813-5b6496e38464.jpg)