bump .NET to v4.8, reduce directory searches

[molsonkiko]

Would close issues #40 and #39.

1. Update .NET Framework to 4.8. This may not be necessary, but I can't compile this project without it. If you want to change this back to 4.0 before your next release, fine by me.
2. Add a check if Notepad++ is shutting down before updating file list (NPPN_BUFFERACTIVATED events fire when Notepad++ is shutting down, but the user doesn't care about the file list at that point)
3. When files in the current file's directory are queried, only update those files at the following times:
   • frmNavigateTo is visible AND
   • if no files in directory list exists yet OR
   • when a new buffer is activated OR
   • if the last refresh of the files in directory list was at least N seconds ago (where N is the value of the Settings.secondsBetweenDirectoryScans variable, default 5)
4. Because querying the files in the directory is much faster now, change the search algorithm so that it checks if all words in the search box are in the current file's directory and the menu commands (to be consistent with the behavior for searching tabs)