from_bang is True when the user query contains a bang.
In this case the category is also set to 'none'.
from_bang only usage was in searx.webadapter.parse_specific :
if from_bang is True, then the EngineRef category is ignored and force to 'none'.
This commit also removes the searx.webadapter.parse_sepecific function.
* Fix "?q=test&engines=wikipedia": fix exception
* Fix "?q=test&engines=wikipedia&categories=images": now the engines from images category are included.
* Fix parse_timeout: make sure a value is always returned
* Various typing fixes (searx.webadapter, searx.search.SearchQuery)
was previously a Dict with two or three keys: name, category, from_bang
make clear that this is a engine reference (see tests/unit/test_search.py for example)
all variables using this class are renamed accordingly.
