Context menu fixes

- Fixes context menu showing up when scrolling (temporary fix until move
to native webworks is complete)
- Adds function bb.getCurScreen to get the current screen element
- Adds property bb-data-useid for menus to set an ID on the element
which makes it easier to retrieve it later
Changed getCurScreen to querySelector
Added imagelist support and removed screen menu changes

- Context menu now also fixed on image lists
- Screen menu changes reversed
Merge pull request #728 from gtoniser/master

Merged this in, I'll update the other files and push up their changes