Backport PR #2294: inputhook_qt4: Use QEventLoop instead of starting up the QCoreApplication I referenced this branch in #2080 and was letting it sit for a little while, but I have decided to make it a full pull request to get some additional visibility. Essentially our Qt event loop mechanism repeatedly starts and quits a `QCoreApplication` object. Unfortunately the `QCoreApplication::quit` slot has a lot of unintended side effects (like emitting an `aboutToQuit` signal which closes all open file dialogs). For our input hook, we _might_ be able to get by with just using a `QEventLoop` whose quit slot is much simpler and less destructive. For a little bit of background on why one might want to just use `QEventLoop::exec`, let's examine what `QCoreApplication::exec` does: ```c++ int QCoreApplication::exec() { if (!QCoreApplicationPrivate::checkInstance("exec")) return -1; // ... [some assertions] threadData->quitNow = false; QEventLoop eventLoop; self->d_func()->in_exec = true; self->d_func()->aboutToQuitEmitted = false; int returnCode = eventLoop.exec(); threadData->quitNow = false; if (self) { self->d_func()->in_exec = false; if (!self->d_func()->aboutToQuitEmitted) emit self->aboutToQuit(); self->d_func()->aboutToQuitEmitted = true; sendPostedEvents(0, QEvent::DeferredDelete); } return returnCode; } ``` As far as I can tell, it's a small wrapper around `QEventLoop::exec` which also: * Sets some variables regarding the current status * Emits an `aboutToQuit` signal right before the function returns (which is the root cause of @denisri's problem in #2080). Historically, our Qt event loop is a python implementation of the (win 32) input hook supplied with the PyQt4 source (see qtcore_input_hook` in `python-qt4/sip/QtCore/qcoreapplication.sip`), which more or less dates to a [mailing list post](http://www.riverbankcomputing.com/pipermail/pyqt/2007-July/016512.html) from July 2007.