plugin config - default activity setting

- this can be set to make the new 'Start tracking for issue' function a
  one click operation

lib/stopwatch.rb

@@ -1,2 +1,36 @@
+# frozen_string_literal: true
+
 module Stopwatch
+  def self.settings
+    Setting.plugin_stopwatch
+  end
+
+  def self.default_activity
+    if id = settings['default_activity'].presence
+      if id.to_s =~ /^\d+$/
+        TimeEntryActivity.find_by_id id
+      else
+        id
+      end
+    end
+  end
+
+  def self.default_activity_for(time_entry)
+    default = Stopwatch.default_activity
+    return nil if default == 'always_ask'
+
+    project = time_entry.project || time_entry.issue&.project
+
+    if project.nil?
+      activities = TimeEntryActivity.shared.active
+    else
+      activities = project.activities
+    end
+
+    if default == 'system'
+      activities.detect(&:is_default?) || activities.detect{|a| a.parent&.is_default?} || (activities.one? && activities[0]).presence
+    else
+      return activities.detect{ |a| a == default || a.parent == default }
+    end
+  end
 end