EditTagDialog: Make reset feedback work by calling

`set_reset_button()` in `UpdateModifiedField()` and catering for
  non-text in `IsValueModified()` (-1 original being same as 0);
  use new `ExtendedEditor::set_font()`;
  connect reset for "rating".
  Make "comment" `tabChangesFocus` to keep tab chain.
ExtendedEditor: New `set_font()` to get emboldened font to work and
  make reset feedback work for `CheckBox` and `RatingBox` by
  overriding `Resize()`.
RatingWidget: Allow tabbed focus and implement keyboard input.

src/dialogs/edittagdialog.ui

@@ -944,7 +944,7 @@
               <item row="15" column="1">
                <widget class="CheckBox" name="compilation">
                 <property name="has_reset_button" stdset="0">
-                 <bool>false</bool>
+                 <bool>true</bool>
                 </property>
                 <property name="has_clear_button" stdset="0">
                  <bool>false</bool>
@@ -1011,6 +1011,9 @@
                   <verstretch>0</verstretch>
                  </sizepolicy>
                 </property>
+                <property name="tabChangesFocus">
+                 <bool>true</bool>
+                </property>
                 <property name="has_reset_button" stdset="0">
                  <bool>true</bool>
                 </property>
@@ -1072,7 +1075,20 @@
                </widget>
               </item>
               <item row="16" column="1">
-               <widget class="RatingBox" name="rating" native="true"/>
+               <widget class="RatingBox" name="rating" native="true">
+                <property name="maximumSize">
+                 <size>
+                  <width>140</width>
+                  <height>16777215</height>
+                 </size>
+                </property>
+                <property name="has_reset_button" stdset="0">
+                 <bool>true</bool>
+                </property>
+                <property name="has_clear_button" stdset="0">
+                 <bool>false</bool>
+                </property>
+               </widget>
               </item>
              </layout>
             </item>
@@ -1217,6 +1233,7 @@
   <tabstop>grouping</tabstop>
   <tabstop>genre</tabstop>
   <tabstop>compilation</tabstop>
+  <tabstop>rating</tabstop>
   <tabstop>fetch_tag</tabstop>
   <tabstop>comment</tabstop>
   <tabstop>lyrics</tabstop>