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()
Fix #21039 - Inspector: VBox height, HBox width and Spacer gap have wrong values

Fixed by changing the type of P_BOX_HEIGHT, P_BOX_WIDTH and P_SPACE properties
update instruments.xml with electronic instruments
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
standardize dialog/menu state interaction
Merge pull request #347 from mgavioli/Fix_21039_wrong_sizes_in_inpector

Fix #21039 - Inspector: some elements have wrong sizes
fix #20320