Add tooltips to the UI

• Where:
  • For all interactive UI controls (buttons, fields, radio buttons, check boxes)
  • Especially important: Preferences pages and icons
• Who:
  • Developers for each plugin/area (add Leena as reviewer)
  • Who will handle less actively maintained and/or contributed plugins?
  •  
• It would be good to have more detailed guidelines for writing tooltips: https://doc.qt.io/qtcreator-extending/qtcreator-ui-text.html#writing-tooltips
  • Start with a verb phrase (for an action, such as most check boxes and radio buttons and menu items) or the description of the thing (for a thing, such as Name)
  • Describe what the control does, not how to use it (if you have to do that, the design is bad and you're most likely using the wrong type of control)
• While we're at it, it would be good to check that the current tooltips are valid and up-to-date
• We could start enforcing tooltips for all new interactive UI controls