While working on another PR related to this I noticed that my VS
generates `.vcxproj` files that are a bit different to the ones we have.
This commit is a quick search & replace of all our project files to make
(primarily) their `ToolsVersion` more in line with what VS does itself:
No `ToolsVersion` for `.vcxproj`, `ToolsVersion="15.0"`
for `.csproj` and `ToolsVersion="4.0"` for `.filters` files.
b0396f1741