fix: calling showWindow() prevents menubar window from closing

[quanglam2807]

This PR fixes the following bug:

Bug Description:

this._isVisible is only set to true in clicked() so if showWindow() is called directly, this._isVisible is not updated and prevents closeWindow() to work properly.

Step to Reproduce:

const { menubar } = require('menubar');

const mb = menubar();

mb.on('ready', () => {
  mb.showWindow();
});

The menubar window will show up but clicking outside would not close the window.

[amaury1093]

Yes, makes sense. Thanks a lot!