Added modal parent to Save/Discard prompts

[omgtehlion]

This fixes the following problem:

When user switches to another note, or presses [Esc] key, a Save/Discard/Cancel dialog appears. If the user at this time switches to another application, then it would not be possible to switch back to mindforger main window by alt-tab or clicking on it. It is possible only by switching exactly to the the prompt.

This fixes it by providing a proper modal parent to these two dialogs, as most other prompts already have.