Add output file path validation in sequence generation dialog

Tested on mac:

• If Output File is a space:
  [DETAILS] QFSFileEngine::open: No file name specified
  [TRACE] Promoting task {Save document} to 'Finished', error 'Can't create tmp file'
  [ERROR] Failed to detect file format: /Applications
  [TRACE] Promoting task {Generate sequence task} to 'Finished', error 'Can't create tmp file'
  [ERROR] Task {Generate sequence task} finished with error: Can't create tmp file
• If Output File is '[': The sequence is saved in Applications/[
• If Output File is ':':
  [TRACE] Promoting task {Save document} to 'Prepared', error 'No permission to write to '/' file.'
  [TRACE] Promoting task {Save document} to 'Running', error 'No permission to write to '/' file.'
  [TRACE] Promoting task {Save document} to 'Finished', error 'No permission to write to '/' file.'
  [DETAILS] Adding document to the project: /
  [TRACE] Promoting task {Generate sequence task} to 'Finished', error 'No permission to write to '/' file.'
  [ERROR] Task {Generate sequence task} finished with error: No permission to write to '/' file.