If the user is typing in the cell editor and has the cursor in a
function call, try to show a tip for the arguments of that function:
(cursor denoted by `|`)
```
sum(|
```
should show:
```
sum(cell names)
```
in a tooltip-like window below the editor.
...and don't let them leak out of their evaluation contexts.
Also keep the exceptions separate from the actual values.
This greatly reduces the number of assertions hit while entering random
data into a sheet.
This commit just moves some code around:
- Give Cell its own file
- Pull all forward-declared classes/structs into Forward.h
- Clean up the order of member functions a bit
