Synfig Animation Studio

Paste canvas have an option 'Canvas', where usually names of all available
exported canvases listed. But at some conditions this list is limited to
just one item.

Steps to reproduce (SVN2316):
* Create a few exported canvases named 'd00', 'd01', 'd02', etc
* New Layer -> Other -> Paste canvas. Select created layer
* Enter animated mode
* Goto '0f'
* Change 'Canvas' parameter to 'd00'
* Exit animation mode
* Encapsulate the paste canvas you created (it is named [d00] now)
* Select [d00] layer again
* Enter animated mode
* Try to change 'Canvas' parameter again. Notice list will contain only
'd00' and 'Other...' elements.