panel: Split away panel handling

This reduces ServerContextService to a mere handler of "docked mode" gsetting.

src/panel.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include "eek/layersurface.h"
+#include "src/layout.h"
+#include "src/submission.h"
+
+// Stores the objects that the panel and its widget will refer to
+struct panel_manager {
+    EekboardContextService *state; // unowned
+    /// Needed for instantiating the widget
+    struct submission *submission; // unowned
+    struct squeek_layout_state *layout;
+
+    PhoshLayerSurface *window;
+    GtkWidget *widget; // nullable
+
+    // Those should be held in Rust
+    struct wl_output *current_output;
+};
+
+struct panel_manager panel_manager_new(EekboardContextService *state, struct submission *submission, struct squeek_layout_state *layout);