Close plugins when image widget closes
Clear dashboard when window closes

Closes #518
test fix
More contracts
Add size, position kwargs to GlueApplication.start

Closes #469
Update window titles when layers are removed

Closes #517
test fix
typo. Closes #523
Power through WCS nonconformity

Closes #527
Add dialog box for setting vmin/vmax

Closes #521
Recover from invalid wcs
Custom subset layer actions
Merge pull request #531 from ChrisBeaumont/0.4-sprint

Bugfixes for version 0.4.0 release