Allow for default external application

[wd5gnr]

As discussed in #2091 this is my take on allowing a default external app. The changes are:

1. Modifier state is captured with mouse input and made available "upstream"
2. By default, there is no default program set and until there is, nothing is different than before
3. When launching an external program, there is a new checkbox to make the program the default
4. If a default program is set, by default Alt+Click on the external tool launches with no dialog; Click operates as usual
5. There is an option to swap the Alt+Click behavior with click (e.g., Alt+Click opens dialog, click opens default)

If there is no default Alt+Click and Click are the same.

We may not need to expose the run in terminal option to the user?

Could probably use better popout text on the help for the option

There are a few other misc changes in here to clean up some of the options stuff from the XYWH change; also a quick write-up on how to add a new option.

Note there is also a new request for the Alt+Click that is consumed as a click by anything else. However, it would be possible to add shift, click, control modifiers to other tools using this much more easily.