Fix #21040: closing the Synth Control window with its own close button leaves the "View | Synth" menu item checked.

Fixed by connecting SynthControl::closeEvent() with the relevant action's setChecked()
Merge pull request #346 from mgavioli/Fix_21040_View_Synth_check_not_synched

Fix #21040: "View | Synth" menu item check not synched with synth window visibility