Fix to able to recover from GraphicsModeException in the override OnH… (#658)

Fix to able to recover from GraphicsModeException in the override OnH… (#658)

* Fix to able to recover from GraphicsModeException in the override OnHandleCreated.

* Changes according to comments:
- m_implementation and m_context is now always set. To DummyGLControl if CreateContext fails
- Changed FailedCreateContext -> HasValidContext
- Changed all if{} to short style with braces.
- Added null propagation when applicable
- Renamed member variables to have 'm_' infront.
- and more ...

* Changed 'm_' prefix into '_'
Added braces to be using Allman Style

* Fixed:

  2) SA1003: GLControl.cs(226,47): Operator '(IGraphicsContextInternal)' must not be followed by whitespace.
  3) SA1003: GLControl.cs(412,49): Operator '(float)' must not be followed by whitespace.

* Removed Paket.Restore.targets from sourcecontrol
Added *.Restore.targets to .gitignore

* Fixed comments
Added *.DotSettings to .gitignore

* Removed *.DotSettings from GitIgnore
Moved *.Restore.targets to Paket section