Analysis:
This bug is there for following reasons.
1.ElectrodeButton for channels is directly added to the ChannelMappingEditor component.
2.When we drag the button we set the visibility of the lastHoverButton which makes it visible and shows on top of other components.
3. Also finding the correct button Logic is not proper as we are not considering the buttons which are scrolled up and invisible.

Solution:
1. Create Viewport for holding the ElectrodeButton for channels.
2. Also create a Holder Component to hold all ElectrodeButton.
3. Add the ElectrodeButton to Holder Component and set the size of the Holder Component to fit all buttons.
4. Set setViewPosition for Viewport based on the scroll and drag.

More Improvement:
1. We can remove upButton and downButton.
2. Viewport has its own scrolling which can be used for scrolling. Currently its been hidden using code "electrodeButtonViewport->setScrollBarsShown(false,false,true,true);"

Ensure that plugin entry points are always exported on macOS and Linux (otherwise, plugins won't load in macOS release builds)

* Refactoring applied mostly because we need to satisfy the new state
  of GenericProcessor after refactoring;
* Also there were a lot of stylystic fixes applied, some minor
  improvements using C++11.

On OS X, the default directory for files explicitly saved/loaded by the user is now ~/Documents/open-ephys.  Added CoreServices::getDefaultUserSaveDirectory(), which returns ~/Documents/open-ephys on OS X and the current working directory on other platforms.

Created Xcode projects for all currently-built plugins.  Added Xcode workspace with "All" scheme for building GUI and all plugins.  Addressed some compiler warnings in plugin code.