Context menu: cleanup & initial reorganisation

This is some initial work on cleaning up the context menu a bit. More work will follow but that will require a lot more refactoring so I'd like to get this reviewed and merged before I start looking into refactoring/reorganising the more complex and involved context menu actions.

Here's a list of what I've changed in this PR:
* moved "Update library" and "Stop scanning" out of the context menu (as it's not a context-aware action, at least not on the item level) and into the video/music window as a control. In confluence the control is a button with a different label and action depending on the state of the video/music scanner.
* moved "Now playing..." out of the context menu (as it's not a context-aware action, at least not on the item level). It's now purely skin specific and does not have a hardcoded control in the video/music view. In confluence it's shown as part of the "Now playing" group in the sidebar.
* fixed "Set content" showing for library folders like genres, actors etc
*moved the "Stack" and "Flatten" toggles out of the video/music window into GUI settings (they already were settings but hidden). I know this will cause a lot of complaints but IMO these are settings you change once and be done with it so no need to have them in the sidebar all the time and confuse users who have no idea what they do. Furthermore having them in the settings allows us to display a description explaining their functionality.
* added a new "Manage ..." item to the context menu for video library items. Using this context menu item will open a (sub) context menu with specific items to manage/alter/adjust the metadata of a specific video library item.
* moved the following context menu items into the "Manage ..." sub context menu: "Edit title", "Link movie to tvshow", "Remove link to tvshow", "Mark as watched", "Mark as unwatched", "Remove episode bookmark"
* added a new "Edit sort title" sub context menu item to the "Manage ..." sub context menu. Ever since I stopped using NFOs the "<sorttitle>" tag was the only reason I still had to manually create NFOs for certain movies so that they would appear in the proper order in my library.

As mentioned earlier there are more things that can be moved into the "Manage ..." sub context menu (e.g. "Choose art" or "Remove from library" etc) but those will require more and complex refactoring.

I have moved/implemented all the code necessary for the "Manage ..." sub context menu in CGUIDialogVideoInfo so it could also be used from the video info screen if properly hooked up in the code and the skin. That could also solve the problem of skinners not having enough space in the video info screen to show all the options/actions they'd like.

Thanks to @ronie for taking the time to make up for my bad/missing skinning skills.