Add prev/next modification commands to the context menu

This is for visibility only as it shows the keyboard shortcut.

Fixes #3

Context.sublime-menu

@@ -3,5 +3,7 @@
 	{ "caption": "Compare with...", "command": "sbs_compare" },
 	{ "caption": "Mark selection for comparison", "command": "sbs_mark_sel" },
 	{ "caption": "Compare selection", "command": "sbs_compare", "args": { "compare_selections": true } },
+	{ "caption": "Previous modification", "command": "sbs_prev_diff" },
+	{ "caption": "Next modification", "command": "sbs_next_diff" },
 	{ "caption": "-" }
-]
+]

sbs_compare.py

@@ -665,10 +665,16 @@ def sbs_scroll_to( view, prev=False ):
 	view.window().show_quick_panel( [ msg ], None )		
 
 class sbs_prev_diff( sublime_plugin.TextCommand ):
+	def is_visible(self):
+	    return self.view.settings().get("is_sbs_compare", False)
+
 	def run( self, edit, string='' ):
 		sbs_scroll_to( self.view, prev=True )
 
 class sbs_next_diff( sublime_plugin.TextCommand ):
+	def is_visible(self):
+	    return self.view.settings().get("is_sbs_compare", False)
+
 	def run( self, edit, string='' ):
 		sbs_scroll_to( self.view )