SourceView: prepare context menu for GLib.Menu

[danirabbit]

We can't use GLib.Menu here yet because SourceView gives us a Gtk.Menu, but we can make sure that we're only using what's available to us in GLib.Menu so that porting will be less painful later. Also do some cleanup of strings while we're here. I can break that into a separate branch if you'd like.

• Make sure all items use action_name instead of manually connecting to activate
• Don't set sensitive on menu items. Ideally we really should set actions enabled/disabled in the document because we also have to worry about keyboard shortcuts, but for now we can consistently use the workaround of removing the action name. Menu items with no action name will automatically be insensitive.
• Add all the menu items together and then set action sensitivity at the end. Makes it easier to see the order the menu is constructed in and make sure we're not conflating widget construction with logic

Menu item renames:

• "Sort Selected Lines" → "Sort Lines". Other menu items like "Copy" or "Cut" don't have the qualifier "Selected". It's understood that context menus operate on the selection. Keep the qualifier "Lines" so it's clear this doesn't sort characters
• "Mark Current Line" → "Mark Line". Same logic. We don't need to specific that context menus operate on the selection
• "Goto Previous Edit Mark" → "Previous Mark". "Goto" is not an English word, but we also don't need to add "Go to" to navigation items. Other navigation items just say "Back" or "Forward". We also don't use the phrase "Edit Mark" when marking the line, it's just "Mark", so mirror that here.

[jeremypw]

Makes sense and everything seems to still work