[BUG #8504] qx.Mobile List Group Header should be sticky
* Added a sticky mixin to common SCSS
* used that sticky mixin inside qx.Moblie List headers.
[BUG #8327] Fixed drag & drop scrolling for nested scroll areas
[BUG #8500] Fixes test with impossible success in qx.ui.table.model.Filtered class
[BUG #8504] qx.Mobile List Group Header should be sticky
* Default to position static if sticky is not supported, for keeping sure "top" does not move all elements.
[BUG #8505] qx.Mobile Scroll container should fire momentumStart/momentumEnd event
* Added detection of momentum, and firing "momentumStart"/"momentumEnd".
Refactored _getParentScrollContainer to be able to be included as a Mixin "MScrollHandling"
Fixed lint warnings
Minor
[BUG #8504] qx.Mobile List Group Header should be sticky
* Special treatment of first group-item because sticky position and the border radius.
[BUG #8485] Point the user to the browser search
[BUG #8481] improved handler disposal check
[BUG #8327] Also respect the current widget for scrolling.
Merge pull request #91 from woprandi/master

[BUG #8500] Fixes test with impossible success in qx.ui.table.model.Filtered class
Minor
[BUG #8428] Added some unit test to reproduce bug.
[Bug #8513] Wrong pdf Activex detection fixed by using qx.bom.client.Plugin#getActiveX, which handles IE11 behavior of artificially hiding the ActivexObject

Furthermore detection of version for pdf Activex improved.
[BUG #8428] Remove all bindings on dispose.
[BUG #8486] first attempt to leaner qx.core.Environment usage in toolchain (i.e. without using _checksMap)
[BUG #8486] empty _checksMap temporarily
[BUG #8428] Added unit test