Issues collection for spreadsheets

This issue collects issues for sheets. They are not very polished at the moment. Example:

[ ] The creation of sheets in libraries and tests is very unclear. You have to use the Build sheet intention. This should be replaced by a button. The intention exists two times, once in the SHEET group and the second time without a group.
[x] You can't delete rows or insert new rows.
[ ] Sheets are part of the language org.iets3.core.expr.repl. They should probably be moved.
[ ] Shortcuts of actions such as F3 to highlight outgoing dependencies are very strange.
[ ] CoordCellRefs such as $S2/B1 in combination with named sheet finders can not be any more confusing to use. Do the scopes that refer to sheets work at all?
[ ] New sheets have the current milliseconds appended as a name. I would remove this suffix.
[ ] Cell args such as the following example have a syntax where I can't even differentiate the input from the output. The syntax also looks very similar to cell labels.
```
(wm: WorkMonth)           
------------------------------
c|  wm.month + "/" + wm.year  
```
[ ] R|in a cell indicates that the cell is a result cell. Let's replace C|instead or find another solution. For me, it is completely unclear what that means.
[ ] CoordCellRefs need better auto-completion support. They can refer to other neighboring cells with 'l', 'r', 'u', and 'd' but there is no code completion for that. Coordinates are also not listed in the code completion but they are at least validated after you enter them.
[ ] Flags and colors in templates are not self-explanatory. What does the orange color mean? What's internal (explanation: the cell can't be referenced)? The style of the collapsed property cell also doesn't fit the rest of the sheet. The same additional properties are in the inspector for non-template sheets.
[ ] Not all intentions are in the SHEET group such as Add Style intentions.
[ ] The show values and booleans are checks are not explained at all and should probably only be visible in the inspector.
[ ] Why are the quote expressions needed when you can refer to other cells with CoordCellRef?
[ ] The SheetEmbedExpr uses weird angle brackets to refer to link the embedded sheet, for example: <SimpleNumberCalculator>. I would use the keyword embed to make it clear what's happening.

IETS3 / iets3.opensource

Issues collection for spreadsheets #692