Make the "Edit Current Profile" dialog modal

This should prevent opening two instance of the EditProfileDialog in the
same process, i.e. if "run all konsole windows in a single process" option is:
- Enabled, then opening the dialog will block user interaction with all
  other konsole windows (including tabs).
- Disabled, then open the dialog will block user interactin with all
  other tabs in the same window

This simplifies the code since it checked if such a dialog was open
somewhere else to prevent crashes.