Improve virtual theme fixer

[tangar76]

Troubleshooting section in Magento Stores Configuration checks for virtual themes.
Improve this awesome feature. Check not only if theme is virtual but also if theme has actual files.

Mark theme as "virtual" if theme source files exist but theme itself has flag virtual in DB.
Mark theme as "removed" if there are no source files of theme and it has flag virtual in DB.

[tangar76]

Issue is "inspired" by today's ticket from one of our clients - "Some product images are missing on storefront". Text below describes why it happened:

The reason of the of missing images was exception during image resize "Required parameter 'theme_dir' was not passed".
Client had Breeze Blank and Breeze Evolution theme in Magento Admin grid. But these themes did not have source files. It looks like they were removed. But records about theme still existed in DB table. And as a result is caused exception during image resize.