[Plugin installer] fix #8876
API documentation for QgisInterface
Fix loading composer item frame thickness (fix #8866)
Propagate WMS identify GML schema error and show it in results, fixes part of #8724
Fix for #8847, blank entries (menu separators) in Customization dialog tree

- Do not add separator actions to menu action tree; separators could not be toggled on/off with customization anyways
Update timeout of network request, if data is transferred
Up to now, any network connection was killed after a max amount of
/qgis/networkAndProxy/networkTimeout milliseconds. This led to problems
for large or otherwise slow downloads.
This patch circumvents timeouts for as long as data is being
transferred.