add icons
fix items with IconBrowseFileExplorer
use different icon for IconFileTree
FormBrowse: Diff tab: add item "Show in File tree" to context menu
FormBrowse: Diff tab: context menu: fix two captions
DiffContextMenu_Opening: refactoring: rename local variable; do file existence check without using return.
FormBrowse: Diff tab: context menu when several items selected: disable items that are not suitable for multiselection
FormBrowse: basic implementation for "Show in File tree" context menu item click
FormBrowse: "Show in File tree" directly goes to the file instead of expanding the whole tree.
Merge pull request #1850 from feinstaub/feature_diff_goto_filetree

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