addons: allow building addons with type different than xbmc.python.script / xbmc.service
scripts/create_addon: cosmetics. require icon.png and changelog.txt in PKG_DIR
scripts/create_addon: drop 'custom addon versions'

this only makes things more complicated.. all addons should follow
the $ADDON_VERSION.$PKG_REV versioning. including those with own
addon.xml
scripts/create_addon: drop PKG_ADDON_EXTENSIONS

special addons should be handled via addon templates in config/addon/
scripts/create_addon: condider addon's own addon.xml as templates
addons: introduce $PKG_ADDON_NAME.

use as addon name if set, otherwise $PKG_NAME is used, as before