[Lazarus] IDE form visibility bug on Linux

[AlexeyT via Lazarus]

Ubuntu latest, gnome desktop

- open IDE (two windows opened: main and messages, some test proj loaded)
- click "minimize" icon on main wnd
- restore wnd again: click Ubuntu sidebar lazarus icon - previews appear
in small rects - choose main wnd
- main wnd restored
- click "Open" toolbar button on main wnd
==> IDE window and Open dialog hide
or
==> all ok