Recover from closed capture window

[Avasam]

While fixing a crash in #93 . I noticed that when the captured window is closed, AutoSplit stops and the "Capture Window" image becomes black. Crash do happen in games, and some speedrun quit on purpose (Save&Quit), it'd be nice if AutoSplit could recover the window once reopened.
We should also be able to pickup a window that is opened after loading settings.

Now that the Capture Region is properly cleared when capture is lost or non-existant from new settings, let's drop the error that pops up at that moment. Showing it on start is sufficient.

• No pop-up on loading settings with no capture window
  • Pickup window capture
• Don't stop/reset AutoSplit upon loosing capture window
• No pop-up on loosing capture window
  • Recover capture upon re-openning window
• Don't allow selecting desktop.