Launch size, Launch position should each have a "as current window" button

### Description of the new feature

There are two settings that govern the size and position of the Terminal window. Both of these allow manual entry of values, so thank you for that.

Settings > Startup > Launch size
Settings > Startup > Launch position

However, using the *current size and position* of the *current terminal window* would be a super-convenient way to set either or both of these settings.

### Proposed technical implementation details

I suggest adding some affordance to the UI in both of those sections, such as a button, to "set from current window." That's the whole feature ask.

Read the Startup settings UI and the existing launch size and position handling first; trace how the current terminal window exposes its size and position. Add the two “set from current window” affordances, then verify that each populates the corresponding setting and that startup uses the saved values.

Written by the indexing model from the issue text.