[enhancement]: Unified Canvas/Mask mode - change "Erase bounding box" to "Erase masked content"

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Contact Details

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### What should this feature add?

There has been an existing issue for this that went inactive back in December '22 (https://github.com/invoke-ai/InvokeAI/issues/1823), but I feel it is worth mentioning again, as it makes a lot of sense.

In Unified Canvas and while in Mask mode, the button "Erase Bounding Box" should be changed to "Erase masked content".
If you want to inpaint something masked with completely new imagery and not img2img based, you currently need to draw the mask and then erase the underlying image beneath the mask, which can be quite finicky and requires multiple passes of adjusting the mask.

It would be a huge improvement if you could paint in a mask and remove the image data "beneath" that mask with one click.
Changing the "Erase Bounding box (Del/Backspc)" to "Erase masked content" would accomplish that.
I can't think of a use case where you would mask an area and would want to remove the content of the bounding box in this content, so the button could be repurposed.

### Alternatives

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### Aditional Content

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