Modifier key descriptions

When a tool is selected, it would be helpful if the effect of various modifier keys was presented to the user somewhere.

For example, when the thermal tool is selected, if you click on a via the thermal connection is turned on or off. However, if you hold shift when you click on a via, the style of the thermal connection is changed. We should present that modifier key and its effect to the user.

The status line would be an appropriate place to display this information, however, it already seems to be a little crowded.

I'm not sure what other tools have modifiers there are, so perhaps we should collect a list of them here.