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This patch adds a method documentation plugins can use to open requested documentation in an external browser and inform the caller about that. This means that the embedded documentation toolview isn't opened or even initialised when you use the "Show documentation for" link in a QtHelp context help popup.
The idea is to make use of the Assistant's remote control mode, spawning a single instance per session which receives commands through its stdin. I have added a configuration option to en/disable the feature.
Currently there is no GUI for selecting the assistant executable or to provide additional arguments (like a .qhc collection file corresponding to the Qt help directory): when active the feature tries to launch an executable called `kdevelop-qthelp-viewer` from the path. It should not be hard to implement an application picker.
There is only a single lowlevel `showDocumentation` method, which means that the external viewer is always triggered, even when you open the embedded docu toolview and click on a QtHelp link in there.