Remove subject filtering, keep only priority buttons

- Remove subject filter menu and related code
- Simplify filtering to priority groups only
- Keep Remove Filters button for priority reset
- Clean up unused code and UI elements

gotify_tray/gui/designs/widget_main.ui

@@ -171,13 +171,6 @@
              </property>
             </widget>
            </item>
-           <item>
-            <widget class="QPushButton" name="pb_subject">
-             <property name="text">
-              <string>Subject</string>
-             </property>
-            </widget>
-           </item>
            <item>
             <widget class="QPushButton" name="pb_remove_filters">
              <property name="text">