Ignore menu events while modal dialogs are shown in wxGTK.

Ignore menu events while modal dialogs are shown in wxGTK.

Normally menus are disabled in this case but not when running under Ubuntu
Unity, so add an explicit test for this to avoid breaking the program expected
flow of control by allowing the user to show the same modal dialog twice, for
example.

This is not ideal, it would be better to visually disable the menu while the
modal dialogs are shown but is better than nothing.

Closes #14823.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73007 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775