Menu Bar disappears when disabling autohide when autohide is enabled in config file

#### Issue details
When mission planner is started up with the menu bar autohide feature enabled in the saved config file, disabling autohide causes the menu bar to disappear as soon as the mouse cursor is moved away from the menu bar until mission planner is restarted.

This is being caused by a double registration of the MainMenu_MouseLeave() event handler on startup, once when MainV2 is initialised, and a second time when AutoHideMenu() is called whilst loading from the config file.

#### Version
1.3.50

#### Platform
[x] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub


Start by locating MainV2 initialization, AutoHideMenu(), and the MainMenu_MouseLeave() handler, then trace both startup and saved-config loading paths. Reproduce the issue with autohide enabled in the config, disable it, and confirm the menu bar remains visible after the cursor leaves it without requiring a restart.

Written by the indexing model from the issue text.