Polling of available printers in now asynchronous (#264,#256)

- Increased polling interval from 1s to 10s
- Use QtConcurrent::run to initiate a poll asynchronously to main event loop
- Do not initiate a poll if the previous poll has not completed
- Use copies instead of references to mCurrentAvailablePrinters in public API

CMakeLists.txt

@@ -115,7 +115,7 @@ if (MINGW)
    set (CMAKE_PREFIX_PATH ${MINGW_BASE_DIR} )
 endif ()
 
-find_package (Qt6 6.2 REQUIRED COMPONENTS Core Gui Widgets PrintSupport Xml Svg LinguistTools Test)
+find_package (Qt6 6.2 REQUIRED COMPONENTS Concurrent Core Gui Widgets PrintSupport Xml Svg LinguistTools Test)
 
 if (WIN32)
    # Locate Qt directories