#Fix Scrolling of Material and Texture Tabs
#Fix crash on empty tab

Add the ability to change the behaviour when you close the last tab:
- Either a new empty tab is added
- or the whole application exits
#Fix crash on too small viewport

Viewport has now a minimum size
Add Hiding of Viewport Borders in Single Viewmode
#Fix Hide Unused Viewport Separators

In Single and Two Viewmode hide the Separators that are not usable
Merge with acgessler/open3mod/master