Keep viewtype consistent for addons and remove view flickering on add-on launch

Two small fixes:

1. Since XBMC adds a slash to add-ons if they are run from e.g. video add-ons, there are two different entries in the ViewModesX.db. By also adding a slash if an add-on is run via RunAddon / RunPlugin, there is just one entry.

2. There was some ugly flickering if an add-on was run via RunAddon etc. which was caused by ```g_windowManager.ProcessRenderLoop()``` in the ```WaitOnScriptResult``` method. Removing it fixed the issue. Everything works just fine but I don't know if there may be some special cases where it has to be there. -> https://github.com/xbmc/xbmc/pull/4189