Fix for sketches script usage in checker options

Reduced blocking timeouts in the UI to 250ms and full block to 5000ms.

Fix for nested lambda tooltips not showing if they were generic
functions.

Refactored some common symbol parsing code so it can be reused rather
than copied.
Merge branch 'master' of git://github.com/fsharp/fsharpbinding into sketches_fixes
Updated addin.xml to ensure tooltips are only applied to the F# mimetype
Removed unnecessary file extension checks.
Removed ExtendsEditor check for FSharpTextEditorCompletion as the mime type constrains this.
Added SupportedFileName and IsAScript

This reduces the footprint of changes and also moves the
Path.GetExtension calls to one place.
Ammended GetDeclaration calls to use blocking timout
Dropped casing on enviromental variables
Updated environmental names prefixed with component name
Updated appveyor.yml environment to allow for slower processing during CI
Merge pull request #871 from 7sharp9/sketches_fixes

Fix for sketches script usage in checker options