gui: single instance mode.

3rd/SingleApplication-3.5.2/singleapplication.pri

@@ -0,0 +1,20 @@
+QT += core network
+CONFIG += c++11
+
+HEADERS += $$PWD/SingleApplication \
+    $$PWD/singleapplication.h \
+    $$PWD/singleapplication_p.h
+SOURCES += $$PWD/singleapplication.cpp \
+    $$PWD/singleapplication_p.cpp
+
+INCLUDEPATH += $$PWD
+
+win32 {
+    msvc:LIBS += Advapi32.lib
+    gcc:LIBS += -ladvapi32
+}
+
+DISTFILES += \
+    $$PWD/README.md \
+    $$PWD/CHANGELOG.md \
+    $$PWD/Windows.md