Add StringFormat option to text cells.

[grokys]

This is similar to #283, but I used a slightly difference approach as the feature in that PR caused problems when text cells were edited.

The differences are:

• The formatting is done in the TDG model layer (TextCell) rather than in the view layer (TreeDataGridTextCell) - this allows us to keep the edited text and the underlying value separate. This prevents the exception when editing.
• The value is now only written to the bound model when editing ends. This prevents exceptions while editing the value (previously worked around in fix the empty cell value edit crash #276
• The default culture is now CurrentCulture rather than InvariantCuture. This aligns with standard Avalonia bindings

[giard-alexandre]

Thanks for looking into this! Just one small potential suggestion to make things easier for consumers. Not blocking at all 🤷

[giard-alexandre]

I guess this won't be possible since ITextCellOptions is not generic and passing in the type of the property for the column but I thought I'd bring it up just in case you see a way to make it happen!
It would be nice to decorate the property with [StringSyntax(StringSyntaxAttribute.NumericFormat)] or any of the other derivatives.

Again, I don't think this would work HERE but it would be cool to be able to specify the stringsyntax somehow.

[grokys]

Hmm, couldn't the string format be anything at all though? I can imagine it being using for displaying arbitrary strings in the cells, e.g. "{0} apple(s)". The StringFormat in standard Bindings can certainly be used that way.

[giard-alexandre]

Of course! I was thinking of having it set according to the TModel type, but you're right that the possibilities are so varied, I don't think it's worth going through all this trouble.

[giard-alexandre]

I just saw that we have the TModel here! Might be able to use StringSyntaxAttribute after all?