wlr/taskbar reads on-click* config values (close, minimize, maximize,
fullscreen, minimize-raise, activate) directly as internal actions in
Task::handle_clicked, but never adopted the eventActionMap_/doAction
mechanism. As a result AModule::handleUserEvent additionally forkExec-ed
the same value as a shell command, e.g. on-click-middle: "close" ran the
action and then failed with "sh: line 1: close: command not found".
Register the taskbar built-in action names in eventActionMap_ so they are
recognized as module actions, and skip the shell forkExec in
handleUserEvent when the configured value is a recognized module action.
Non-action values are still run as user shell commands.
Fixes#3284.