CSS: Don't throw exceptions on HTTP URLs

Only call Directory.Exists() if it's actually a local path.
CSS: Fix image preview tooltips

Allow HTTPS images
Add better exception handling
Remove duplicated code
Use centralized relative path logic

Fixes #255
Helpers: Misc Project Path Fixes

Fixes #232 (GetRootFolder() for Installer projects)
Fix path methods for Miscellaneous Files with no solution open
CSS: Load image tooltips asynchronously

Also handles load failures correctly, and
shows image dimensions in base64 tooltips.
Merge pull request #263 from SLaks/css-quick-info-rewrite

Completely rewrite CSS Image Tooltips