FE/Qt: Place the full-screen mini-toolbar on the machine window's monitor

[dr1fter]

Fixes #842.

Problem

In full-screen mode the mini-tool-bar's outer window can be placed on a
different host monitor than the machine window it belongs to. That window is
screen-sized and carries _NET_WM_STATE_FULLSCREEN, so the window manager
treats the unrelated monitor as holding a full-screen window and desktop panels
on it stop being painted for as long as the VM runs.

Observed with one guest screen on a three-monitor X11 host: the machine window
is correctly on DP-2 (+1920+0) while the tool-bar lands on DP-4 (+3840+0), and
VBox.log records Move mini-toolbar for window #0 to 0x0.

Cause

UIMiniToolBar::sltAdjust() resolved its host screen with
UIDesktopWidgetWatchdog::screenNumber(m_pParent), which goes through
m_pParent->windowHandle()->screen(). During the full-screen transition that
native association can still lag behind the geometry already assigned to the
top-level widget by UIMachineWindowFullscreen::placeOnScreen().

The surrounding code already treats this area as racy — the existing
WORKAROUND comment just below handles screenNumber() returning -1.

Change

Resolve the screen from the parent's geometry instead, using the existing
screenNumber(const QPoint &) overload. sltAdjust() is queued from the
parent's show event, i.e. after placeOnScreen() has moved the parent, so its
geometry is authoritative at that point.

Validation

Arch Linux, X11, Cinnamon 6.6.9/Muffin 6.6.3, three 1920x1200 monitors,
VirtualBox 7.2.16, Qt 6.11.2. Guest with monitorcount=1,
GUI/Fullscreen=true, mini-tool-bar enabled.

	stock	patched
logged tool-bar move	0x0	1920x0
tool-bar window position	3840,0	1920,0
machine window position	1920,0	1920,0
host monitors full-screen	two	one
unrelated desktop panel	not painted	painted

To reproduce without the patch: start a single-screen guest full-screen on a
multi-monitor host where Qt's screen order differs from the window manager's,
and observe the panel on another monitor stop being drawn. Setting
GUI/ShowMiniToolBar=false makes it return, confirming which window is
responsible.

[oracle-contributor-agreement]

Thank you for your pull request and welcome to our community! To contribute, please sign the Oracle Contributor Agreement (OCA).
The following contributors of this PR have not signed the OCA:

• PR author: dr1fter
• ccwienk@dr1fter.de (@dr1fter)

To sign the OCA, please create an Oracle account and sign the OCA in Oracle's Contributor Agreement Application.

When signing the OCA, please provide your GitHub username. After signing the OCA and getting an OCA approval from Oracle, this PR will be automatically updated.

If you are an Oracle employee, please make sure that you are a member of the main Oracle GitHub organization, and your membership in this organization is public.