Merge all validation files at one place

[pictos]

Today, our validations task #81 is split per project, we should merge them into one .target file for better modification and visibility.

Other enhancements:

• We should be validating all resource sizes here
• We're specifying the same file name (eg dotnet_bot.scale-300.png), and in a lot of cases the same relative path (eg \MyAssets\Nested). Was thinking we could define these once and then use the property everywhere