Fix indicator popovers failing to open from keybindings on Wayland

[mehdip2007]

Fixes #726

Problem

On Wayland, indicator popovers — including the Applications Menu — fail to open when triggered from a keybinding (e.g. the Super accelerator via org.gnome.Shell / D-Bus activation). Clicking an indicator works, but exactly one keyboard-triggered open succeeds after every click. After enough open/close cycles the popover can also end up visible but never drawn, and the panel eventually dies with a Wayland protocol error.

Causes

• xdg_popup grab: the popover was created with autohide = true, which makes GTK request an xdg_popup grab. Wayland only grants that grab against a recent input event serial, and a popover opened from a keybinding has received no input at all — so the compositor refuses the popup and it is never mapped. Clicking an indicator supplies a serial, which is why one keyboard-triggered open succeeded after every click.
• Reparenting on every cycle: unparent () + set_parent () on every open/close destroys and recreates the popover surface, eventually leaving it visible but unmapped and ending in a Wayland protocol error.
• get_panel () requested twice: realize can fire more than once; a second get_panel () request on the same surface makes the compositor terminate the client with a protocol error, taking the whole panel down.
• No keyboard focus: panels are never focused automatically by the compositor, so typing in the Applications Menu search field never reached the popover.

Changes

• PopoverManager: create the popover with autohide = false and dismiss it when the panel toplevel loses keyboard focus (PopoverManager.close ()), instead of relying on the grab.
• PopoverManager: only reparent the popover when it actually moves to a different indicator, and don't unparent on close.
• PanelWindow: guard get_panel () against a second request and make realize handling run only once.
• PanelWindow: request keyboard focus when a popover opens, with a Wayland roundtrip so the popup is created after focus is actually granted — without this, typing in the Applications Menu search field does not work.

Testing

Built and tested on Wayland: the Applications Menu now opens reliably from the Super key every time (not just once after each click), the search field accepts typing, and repeated open/close cycles across indicators no longer produce an unmapped popover or protocol errors.

[danirabbit]

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