Bug # 2 - Tool Windows disappear if dropped on any middle side glyphs

[kabua]

Using: VS2013 Test Changed: ToolViewModel's _isVisible = true; to false;

1. start app (notice no Views are selected or showing)
2. Click View > Solution Explorer (to show)
3. Move Solution Explorer from the Right-hand side to any middle side glyph (i.e. left, top, bottom, right) the Solution Explorer simply disappears. Note that the middle center glyph works correctly.
4. Click View > Solution Explorer (to hide)
5. Click View > Solution Explorer (to show). It doesn't fix the issue, the tool is now gone forever.

Expectation The Solution Explorer is to be repositioned correctly and not simply disappear forever.