Hook to add hotkey definition texts via plugins

This enables giving custom descriptive texts in the "Keyboard shortcuts help" window for hotkey actions defined by plugins.

Example implementation in "`extends Plugin`" plugin:

	function init($host) {
		$this->link = $host->get_link();
		$this->host = $host;

		$host->add_hook($host::HOOK_HOTKEY_INFO, $this);
	}

	function hook_hotkey_info($hotkeys) {
		$hotkeys[__("Article")]["open_in_background_tab"] = __("Open in background tab (Chrome/Opera only)");

		return $hotkeys;
	}

Without this, hotkeys defined to trigger plugin defined actions won't even show up in the "Keyboard shortcuts help" window since they have no textual representation available.