Support new selected and inactive tab highlighting editor scheme keys

Selected and inactive tabs were previously styled using the
corresponding `ui.EditorTabs.*` UI themey keys. Some of these keys are
now deprecated or marked as "unknown" to the UI theme scheme validator.
The highlighting is now controlled using the new `TAB_SELECTED` and
`TAB_SELECTED_INACTIVE` editor color scheme keys added to the IDE
platform code in JetBrains/intellij-community@bf26eb8.

In my opinion there are some keys that should not be placed in the
editor scheme (including these both) and it feels like there is
inconsistency between the UI theme API and editor color scheme API.
Anyway, in order to style tabs correctly again, both new editor scheme
keys have been added using the same colors like the UI theme keys.

Reported by "Tom1206" (https://github.com/Tom1206) in #120 (comment)
Related to GH-120
GH-126

resources/themes/nord.xml

@@ -58,6 +58,8 @@
     <option name="SELECTED_TEARLINE_COLOR" value="434c5e" />
     <option name="SELECTION_BACKGROUND" value="434c5e" />
     <option name="SOFT_WRAP_SIGN_COLOR" value="4c566a" />
+    <option name="TAB_UNDERLINE" value="88c0d0" />
+    <option name="TAB_UNDERLINE_INACTIVE" value="616e88" />
     <option name="TEARLINE_COLOR" value="4c566a" />
     <option name="VCS_ANNOTATIONS_COLOR_1" value="3b4252" />
     <option name="VCS_ANNOTATIONS_COLOR_2" value="3b4252" />
@@ -4462,6 +4464,18 @@
         <option name="FOREGROUND" value="a3be8c" />
       </value>
     </option>
+    <option name="TAB_SELECTED">
+      <value>
+        <option name="FOREGROUND" value="d8dee9" />
+        <option name="BACKGROUND" value="4c566a" />
+      </value>
+    </option>
+    <option name="TAB_SELECTED_INACTIVE">
+      <value>
+        <option name="FOREGROUND" value="d8dee9" />
+        <option name="BACKGROUND" value="3b4252" />
+      </value>
+    </option>
     <option name="TAG_ATTR_KEY">
       <value>
         <option name="FOREGROUND" value="8fbcbb" />