Declare plugin.video.youtube as a Kodi addon dep

addon.xml now requires plugin.video.youtube >=7.0.0 in addition to
xbmc.python and (optional) inputstream.adaptive. When a user installs
torttube, Kodi resolves the dep and auto-fetches plugin.video.youtube
from the official Kodi addon repository — user only manages torttube,
the dep is transparent.

This is the preferred 'one-addon experience' without the
maintenance liability of vendoring + maintaining 5.6MB / 15k LOC of
their player code in our tree.

README architecture diagram updated to show pv.youtube as a dependency
rather than a sibling. docs/install.md notes that Kodi auto-installs
the dep — no manual pv.youtube step needed.

Addon version 0.0.12.

addon/plugin.video.torttube/addon.xml

@@ -1,10 +1,11 @@
 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
 <addon id="plugin.video.torttube"
        name="torttube"
-       version="0.0.11"
+       version="0.0.12"
        provider-name="Sulkta">
   <requires>
     <import addon="xbmc.python" version="3.0.0"/>
+    <import addon="plugin.video.youtube" version="7.0.0"/>
     <import addon="inputstream.adaptive" version="2.0.0" optional="true"/>
   </requires>
   <extension point="xbmc.python.pluginsource" library="main.py">