Debugger should display "step" buttons next to the active instruction

[hannobraun]

Currently, the step button is displayed above the active functions view. (As of this writing, there is only "Step Into". I expect that we'll also have "Step Over", "Step Return", and eventually more buttons to rewind.)

This is problematic in two ways:

• The button is visible, even if there is no way to use it (as in this screenshot). This is unnecessary and potentially confusing.
• When there is a way to use it, when the process is stopped, then it is physically removed from the location where it has an effect, the active expression.

Both of those issues can be solved by moving the buttons right next to, preferably to the right of, the active expression.

[hannobraun]

This is related to https://github.com/hannobraun/caterpillar/issues/35, which should probably be addressed first, or together with this issue. Otherwise, things might get confusing, if the step buttons are not even visible when relevant.

[hannobraun]

I decided to address this without looking into #35 for now. So much is broken in the debugger anyway right now, and addressing this has helped me unlock some fixes.