Label: Make the meaning of the blank cell visible

On changes dashboard the label columns often have some blank cells
especially when user has some customized label for some special branch.

  [1] some of them mean for current change this label is not applicable.
  [2] some of them mean this label is applied on this change's branch but
      this change has not got any vote on this label yet.

In order to figure out what a blank cell means, a user has to click that row
to open the ChangeScreen and check the approval table.

This commit makes it easy to distinguish these blank cells by setting
blank cells matching case [1] to display grey background and have a title
'Label not applicable'.

Even for the case where all labels are project-specific, this feature is
valuable.

Change-Id: Ic965ad51f60919ecf1d3cefb9456271950d51302

gerrit-gwtui/src/main/java/com/google/gerrit/client/GerritConstants.java

@@ -48,6 +48,8 @@ public interface GerritConstants extends Constants {
 
   String inactiveAccountBody();
 
+  String labelNotApplicable();
+
   String menuAll();
   String menuAllOpen();
   String menuAllMerged();

gerrit-gwtui/src/main/java/com/google/gerrit/client/GerritConstants.properties

@@ -31,6 +31,8 @@ noSuchGroupTitle = Code Review - Unknown Group
 
 inactiveAccountBody = This user is currently inactive.
 
+labelNotApplicable = Label not applicable
+
 menuAll = All
 menuAllOpen = Open
 menuAllMerged = Merged

gerrit-gwtui/src/main/java/com/google/gerrit/client/GerritCss.java

@@ -141,6 +141,7 @@ public interface GerritCss extends CssResource {
   String infoTable();
   String inputFieldTypeHint();
   String labelList();
+  String labelNotApplicable();
   String leftMostCell();
   String lineHeader();
   String lineNumber();

gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeTable2.java

@@ -230,7 +230,8 @@ public class ChangeTable2 extends NavigationTable<ChangeInfo> {
 
       LabelInfo label = c.label(name);
       if (label == null) {
-        table.clearCell(row, col);
+        fmt.getElement(row, col).setTitle(Gerrit.C.labelNotApplicable());
+        fmt.addStyleName(row, col, Gerrit.RESOURCES.css().labelNotApplicable());
         continue;
       }
 

gerrit-gwtui/src/main/java/com/google/gerrit/client/gerrit.css

@@ -595,6 +595,9 @@ a:hover {
 .changeTable .dataCell.singleLine {
   white-space: nowrap;
 }
+.changeTable .dataCell.labelNotApplicable {
+ background: #F5F5F5;
+}
 .changeTable .iconHeader {
   border-top: 1px solid backgroundColor;
   border-bottom: 1px solid backgroundColor;