UI: Rename 'obs' dir to 'UI'

This is to prevent confusion both when prefixing commits and when
reading the directory structure for the first time.

UI/dist/obs.desktop

@@ -0,0 +1,12 @@
+[Desktop Entry]
+Version=1.0
+Name=OBS
+GenericName=Streaming/Recording Software
+Comment=Free and Open Source Streaming/Recording Software
+Comment[ru]=Бесплатная программа с открытым кодом для записи/трансляции видео
+Exec=obs
+Icon=obs
+Terminal=false
+Type=Application
+Categories=AudioVideo;Recorder;
+StartupNotify=true