Add a "symbol" popup to Calc GUI

[rlwhitcomb]

With a palette of all the Unicode symbols that are supported (arranged logically, of course). Things like the "pi" and "e" and "phi" symbols, arithmetic operators, maybe tabs for the built-in function names too (could enter the name, the parens, and position the caret after the opening paren)... Could use different background colors for different types of symbols, and could throw in parens, brackets and braces too, and "=" maybe, and the numbers, just so you could "type" with the mouse in the popup dialog without typing... Can't be too big, though.

Need to find a convenient place for a little button to initiate it (and a suitable keyboard shortcut, maybe Option/Alt-S).... Should be near the "Expressions" label on the bottom Border element.... (can we have a button element as part of the text??)

Watch out for how to position it so it doesn't cover the insertion point. Does it move as text is entered??