Revert "page-wrapper: Use AdwNavigationView backward button"

AdwNavigationView requires a bad style hack to align the button to the
top and also causes issues when pushing new pages onto the stack via
gestures.

This reverts commit 4d2fdd4920.

data/resources/style.css

@@ -7,8 +7,3 @@
   background: none;
   box-shadow:none;
 }
-
-/* Super hacky libadwaita button alignment workaround */
-headerbar .back {
-  margin-bottom: 123px;
-}

data/resources/ui/widgets/page_wrapper.blp

@@ -7,6 +7,7 @@ template $PageWrapper : Adw.NavigationPage {
     Adw.HeaderBar {
       show-start-title-buttons: false;
       show-end-title-buttons: false;
+      show-back-button: false;
       styles ["flat"]
 
       [start]
@@ -19,6 +20,16 @@ template $PageWrapper : Adw.NavigationPage {
           visible: bind win_controls_start.empty inverted;
         }
 
+        Revealer previous_revealer {
+          transition-type: crossfade;
+          child: Button {
+            action-name: "win.previous-page";
+            focusable: true;
+            focus-on-click: false;
+            icon-name: "go-previous-symbolic";
+          };
+        }
+
         Revealer next_revealer {
           transition-type: crossfade;
           child: Button {

src/ui/page_wrapper.py

@@ -126,6 +126,7 @@ class PageWrapper(Adw.NavigationPage):
     __gtype_name__ = __qualname__
 
     next_revealer = Gtk.Template.Child()
+    previous_revealer = Gtk.Template.Child()
     title_image = Gtk.Template.Child()
     title_label = Gtk.Template.Child()
     reload_revealer = Gtk.Template.Child()
@@ -188,7 +189,8 @@ class PageWrapper(Adw.NavigationPage):
         del self.page
         self._set_new_page(self.page_name)
 
-    def update_navigation_buttons(self, is_last: bool):
+    def update_navigation_buttons(self, is_first: bool, is_last: bool):
+        self.previous_revealer.set_reveal_child(not is_first)
         self.next_revealer.set_reveal_child(not is_last)
         self.reload_revealer.set_reveal_child(
             self.page_name in reloadable_pages)

src/ui/window.py

@@ -84,6 +84,7 @@ class OsInstallerWindow(Adw.ApplicationWindow):
         self.shortcut_controller.set_scope(Gtk.ShortcutScope(1))
 
         self._add_action('next-page', self._navigate_forward, '<Alt>Right')
+        self._add_action('previous-page', self._navigate_backward, '<Alt>Left')
         self._add_action('reload-page', self._reload_page, 'F5')
         self._add_action('about-page', self._show_about_page, '<Alt>Return')
         self._add_action('quit', self._show_confirm_dialog, '<Ctl>q')
@@ -187,7 +188,8 @@ class OsInstallerWindow(Adw.ApplicationWindow):
 
     def _update_page(self):
         current_page = self.navigation_view.get_visible_page()
-        current_page.update_navigation_buttons(self._current_is_last())
+        is_first, is_last = self._current_is_first(), self._current_is_last()
+        current_page.update_navigation_buttons(is_first, is_last)
 
     def _load_next_page(self, offset: int = forward):
         page_name = self._get_next_page_name(offset)
@@ -203,12 +205,23 @@ class OsInstallerWindow(Adw.ApplicationWindow):
         previous_page_name = self.previous_pages.pop()
         self._load_page(previous_page_name, offset=backwards)
 
+    def _current_is_first(self):
+        page = self.navigation_view.get_visible_page()
+        return page.get_tag() == self.pages[0]
+
     def _current_is_last(self):
         page_name = self.navigation_view.get_visible_page().get_tag()
         return page_name == self.pages[-1]
 
     ### callbacks ###
 
+    def _navigate_backward(self, _, __):
+        with self.navigation_lock:
+            if self.previous_pages:
+                self._load_previous_page()
+            elif not self._current_is_first():
+                self._load_next_page(backwards)
+
     def _navigate_forward(self, _, __):
         with self.navigation_lock:
             if not self._current_is_last():