Per @Trinitou suggestions, a plugin specific menu item should only be enabled (available to execute by user)

This fixes the underlying cause of the crashes reported in free-audio#40 and free-audio#42

Therefore this also reverts the now obsolete change in previous merged pull request free-audio#43

Upon load change we update the menu items

Tab indentation set to 3 to be consistent with existing code

Note: Follows the QT memory ownership model, and the QAction pointers lifetime is handled by their parents

Testing:
Launch with no plugin loaded, see menu items are greyed out (disabled), user cannot select these items (as intended) therefore code is not executed and app doesn't crash

Launch with plugin loaded, see menu items are available and executed when user clicks, and confirm still no crashes

Images taken for testing

Per @Trinitou feedback:

1. pluginLoadedChanged is now a signal of PluginHost

2. Connect MainWindow:: updatePluginMenuItems to this signal

3. Rename updateMenuItems to updatePluginMenuItems (so it specifies it's these are plugin related menu items)

4. Remove i_prefix in function parameter to follow code style

5. Use default argument false for updatePluginMenuItems