Ubuntu 17.04 launch results in segfault if the build was done
by GCC 6.2 (works fine with GCC 4.9). Backtrace shows that it
crashes in gtk_init_check() call somewhere in libmirclient and
tests show that it works fine with GDK_BACKEND=x11.
So we use gdk_set_allowed_backends() method to explicitly state
that we support only "x11" GDK backend, that way it doesn't try
to use libmirclient and it does not crash.
Fix#3176#3162
ShellExecute() call reenters Qt event loop, so each time we
schedule a delayed action (like destroying FileLoader) and after
that we call ShellExecute (in psOpenFile) we destroy it inside
this call and can't use it after.
So now we perform ShellExecute calls only delayed (using task queue).
A regression was introduced in 1.0.12 version which led to wrong
activations of the main window: if a custom popup menu was shown
for the tray icon or a custom notification was clicked.
