fix: keep window width on KWin/Wayland when adding tabs or toggling visibility

[arazgray]

Problem

On KDE Plasma (KWin) Wayland sessions, where Guake runs as an XWayland client (GDK_BACKEND=x11), the main window does not keep its configured width:

• opening a new tab shrinks the window to the VTE natural width (~80 columns, e.g. 1920px -> 1280px); hiding/showing restores it, and
• (once the tab path is fixed) hiding/showing maps at full width and then gets snapped back to the narrow width asynchronously by the compositor.

Root cause: RectCalculator.set_final_window_rect() (guake/utils.py) is only applied on show(). Adding/removing a tab calls notebook.show_all(), which lets GTK/KWin renegotiate the toplevel to its natural size, and nothing re-asserts the configured size. KWin additionally forces a narrow size on fresh maps after show() returns, so pre-map resizes alone do not stick (verified with configure-event tracing: correct 1920px request logged, window still lands on 1280px).

Changes

• guake/notebook.py: re-apply the final window rect (deferred via GLib.idle_add) after a tab is added (new_page()) or removed (remove_page()) while the window is visible.
• guake/utils.py: floor the toplevel minimum width to the configured width in set_final_window_rect() (cleared on the maximize path) so the compositor cannot shrink below it.
• guake/guake_app.py: in show(), set the rect as the WM map-time default size, flip the DOCK->NORMAL type hint before mapping instead of after, and re-assert the size after mapping; added a debounced configure-event watchdog that restores the enforced width on large unexpected shrinks (skipped when hidden/fullscreen).
• guake/gsettings.py: keep the enforced width in sync when size/alignment settings change.

Testing

Manually verified on Kubuntu-style setup (Plasma 6 Wayland, single 1920x1080 output, window-width=100) with xwininfo: width stays at 1920px across new tabs, tab closes and hide/show cycles. py_compile passes on all touched files.