Minor refactor of the UI

[devoncarew]

After adding support for showing api documentation, the DartPad UI can seem a bit cumbersome. The doc area shows on mouse click - sometimes surprising to users - and resizes the editing area when becoming visible. I'd like to propose a minor refactor:

• we change the doc area to show on mouse hover using flutter tooltips (still rendering into a flutter markdown widget); this tooltip may be positioned relative to the identifier or perhaps in a fixed location (the top 1/3 of the editor? over the execution area?)
• we change the console area to have an unobtrusive, single line separator from the execution area above it; this mirrors the problems view area and avoids that separator looking draggable

These changes would also make it easier for us to add an optional fixed-height area above the editor - a 'create a snippet that does xyz' area.