multi-choice: Bind category to tooltip + a11y label

This adds a tooltip to the multi choice buttons, with the text
"Previous/Next (category)". This fixes the warnings in the date
chooser popover.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/681>

src/gui/common/gcal-date-chooser.blp

@@ -12,13 +12,10 @@ template $GcalDateChooser: Adw.Bin {
       animate: false;
       value: 0;
       wrap: true;
+      category: _("Month");
       visible: bind $get_split_choice_visible(template.show-heading, template.split-month-year) as <bool>;
       notify::value => $split_multi_choice_changed() swapped;
 
-      accessibility {
-        label: _("Month");
-      }
-
       layout {
         column: 0;
         row: 0;
@@ -32,12 +29,9 @@ template $GcalDateChooser: Adw.Bin {
       max-value: 9999;
       animate: false;
       value: 0;
+      category: _("Year");
       notify::value => $split_multi_choice_changed() swapped;
 
-      accessibility {
-        label: _("Year");
-      }
-
       layout {
         column: 1;
         row: 0;
@@ -52,12 +46,9 @@ template $GcalDateChooser: Adw.Bin {
       popover: popover;
       animate: false;
       value: 0;
+      category: _("Month");
       notify::value => $combined_multi_choice_changed() swapped;
 
-      accessibility {
-        label: _("Month");
-      }
-
       layout {
         column: 0;
         column-span: 2;
@@ -89,10 +80,7 @@ Popover popover {
       max-value: bind month_choice.max-value bidirectional;
       animate: bind month_choice.animate bidirectional;
       value: bind month_choice.value bidirectional;
-
-      accessibility {
-        label: _("Month");
-      }
+      category: bind month_choice.category bidirectional;
     }
 
     $GcalMultiChoice {
@@ -100,10 +88,7 @@ Popover popover {
       max-value: bind year_choice.max-value bidirectional;
       animate: bind year_choice.animate bidirectional;
       value: bind year_choice.value bidirectional;
-
-      accessibility {
-        label: _("Year");
-      }
+      category: bind year_choice.category bidirectional;
     }
 
     styles [