insert/cut frame is not constrained to the canvas size.

Steps to reproduce:

1. add a new animation
2. select the front edge of the last frame on the canvas (as shown in the gif)
3. left-click to insert new frame

Expected behavior:

at the ninth tile, the tool should stop inserting more frames. That or it should make the canvas bigger.

Actual behavior:

The tool doesn't care about how many tiles the canvas consists of.