Canceling a screenshot shall not disable buttons if previous screenshot is visible

Problem: take screenshot, it shows up in Spectacle. Take, for example, new screenshot with rectangluar region. Cancel it, the previous screenshot is still in Spectacle, but most of the buttons are disabled.

Solution: if checking that the new Pixmap has been forwarded to setScreenshotAndShow() is empty, because the 'shot' was canceled, check if a previous screenshot is already visible in Spectacle and set mPixmapExists to true so the buttons don't get disabled. Do not check for mKSWidget->isScreenshotSet() before the 'if (mPixmapExists) {}', in case the empty pixmap will overwrite the old screenshot.