Added support for searching UNC file paths, fix some bugs and set background color of selected file

Added support for searching Microsoft UNC (Universal Naming Convention) file paths.
Make sure Everything is not currently indexing before trying to search with Everything.
Make sure Everything has indexed the drive being searched (so we don't use it to search drives not being indexed by Everything).
Change the background color of the selected file so it's easy to see what was selected after clicking in the search results Rich Text Box.
Fix an issue with no results returned when the "Folder To Search" was the root of the disk (i.e. "C:").
Fix an issue displaying the message "No files found matching the file specification:" when not finding any files that match the filespec.