Add show and hide events to the WindowMenu class

[aarranz]

Proposed changes

This PR adds show and hide events to WindowMenu instances, this way it is possible to detect those changes. In particular, the wirecloud-keycloak plugin wants to use the hide event to detect when the MessageWindowMenup dialog is closed to react.

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• [x] I have read the CONTRIBUTING doc
• [x] I have signed the CLA
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)
• [ ] Any dependent changes have been merged and published in downstream modules

Further comments

Required by wirecloud-keycloak.

[coveralls]

Coverage increased (+0.2%) to 88.906% when pulling adf1a8b7ffd3303975aca949d1fc6353724e733b on aarranz:feature/windowmenu-basic-events into a4b9711098ee8caa3d0e9524c9b405d8356de088 on Wirecloud:develop.