Material-table: Add a handler `onRowEdit` and `onRowEditCancel`

[AntoniusGolly]

For when editable objects are provided...

Is your feature request related to a problem? Please describe.
I check for errors when validating input data on editing or adding new items. Those errors I want to clear when the editing is canceled.

Describe the solution you'd like
There is already some nice handlers as onOrderChange or onRowClick. I would like to have an additional handler onRowEdit when editing is started and onRowEditCancel when the X is pressed in editing mode.

Additional context
Also, I think it makes great sense to have a handler for when the detailPanel is opened/closed.

[hGica]

I am also searching for a way to do this. I also couldn't manage to successfully override editrow component to get the functionality as there is no documentation on how to do this.

[victorwvieira]

It's a great idea, I also need identify when the user is editing.

[hGica]

@waisbek see my answers on https://github.com/mbrn/material-table/issues/751 though I wasn't able to get access to is editing

[CycoPH]

Internally there is the 'onEditingCanceled' event. It just has to be exposed to the outside.
This would make external messaging way easier if we know when cancel was pressed.

[Rombusevil]

I've stumbled upon this issue due to needing functionality to handle a user message when quitting the page with unsaved data. I've made the PR providing a callback that will be called with a boolean indicating a "isEditing" status.
You can try it out installing it like this npm install git://github.com/Rombusevil/material-table.git#onEditingStatusChange-build --save

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