EditableCell behavior improvements

[NEO97online]

I love the Editable Table, but there are some behaviors that feel like bugs, especially in comparison to most spreadsheet software:

• [ ] Cell textarea opens when Shift is pressed (Expected: Textarea should not open with Shift)
• [ ] Caret position starts at beginning of textarea (Expected: Caret should start at end of text)
• [ ] When focusing a cell then pressing a letter, the letter is appended to the end of the text (Expected: pressed letter should replace the existing cell text)
• [ ] When focusing a cell then pressing a number, nothing happens (Expected: The textarea should open, replacing existing text with the pressed number)

These expectations are based on my personal UX expectations and verified against the behavior on http://sheets.google.com.