add clean-up at the end of SceneViewer::contextMenuEvent for OSX

[tomosu]

fixed problem reported in #115 and #73 .

[kogaki]

I feel this approach is a little bit confusing because view-related method contextMenuEvent clean-ups control-related members (m_mousebutton etc ...). If there's no other solutions, it is ok to merge. But here are my suggestions

1. Cleaning-up control-realted members might be done first in the contextMenuEvent, not last
2. Plz add some comments on the code (e.g. cleaning-up after right button click)

[tomosu]

Thank you for your comment.
I reflected your suggestions to sceneViewerEvent.
Please check this.
cb0b4d0

[kogaki]

LGTM

[xenogenesi]

I've a patch for linux which fake a release event, apparently on linux and on macosx the release event is lost handling the contextMenuEvent

[xenogenesi]

posted on #51

About #73 / #115, if someone want to give a try to this (also on mac)

[kogaki]

@xenogenesi Great. Your solution looks more reasonable. Could you create the another PR so that we can test and merge smoothly ?