Linkstate refactor (#609)

* Linkingstate Refactor, and displaying link info in rightpanel

Big refactor to LinkingState
    Collects descriptive/user-facing labels into FilterState
    Unifies/cleans up some logic
    Adds LinkTypeDescription, a string enum which can be used
    to easily switch/case between various cases of linking, and
    codifies the logic in one place (currently only used for linkInfo)

Adds Link info to creator panel, near SelectBy dropdown

Bugfix: Disables linking from Attachment columns
Bugfix/Behavior change: changed linking with empty RefLists to better
match behavior of refs.
    for context: Linking by a blank Ref filters to show records with a
    blank value for that Ref. Previously this didn't work with RefLists. 
    Linking from a blank refList would show no records 
    (except in some cases involving summary tables)
    Fixed this so that linking by a blank val consistently means "show
    all records where the corresponding col is blank"

app/client/ui/selectBy.ts

@@ -42,6 +42,9 @@ interface LinkNode {
   // is the table a summary table
   isSummary: boolean;
 
+  // does this node involve an "Attachments" column. Can be tricky if Attachments is one of groupby cols
+  isAttachments: boolean;
+
   // For a summary table, the set of col refs of the groupby columns of the underlying table
   groupbyColumns?: Set<number>;
 
@@ -114,6 +117,12 @@ function isValidLink(source: LinkNode, target: LinkNode) {
     return false;
   }
 
+  //cannot select from attachments, even though they're implemented as reflists
+  if (source.isAttachments || target.isAttachments) {
+    return false;
+  }
+
+
   // cannot select from chart
   if (source.widgetType === 'chart') {
     return false;
@@ -230,6 +239,7 @@ function fromViewSectionRec(section: ViewSectionRec): LinkNode[] {
   const mainNode: LinkNode = {
     tableId: table.primaryTableId.peek(),
     isSummary,
+    isAttachments: isSummary && table.groupByColumns.peek().some(col => col.type.peek() == "Attachments"),
     groupbyColumns: isSummary ? table.summarySourceColRefs.peek() : undefined,
     widgetType: section.parentKey.peek(),
     ancestors,
@@ -266,6 +276,8 @@ function fromPageWidget(docModel: DocModel, pageWidget: IPageWidget): LinkNode[]
   const mainNode: LinkNode = {
     tableId: table.primaryTableId.peek(),
     isSummary,
+    isAttachments: false, // hmm, we should need a check here in case attachments col is on the main-node link
+    // (e.g.: link from summary table with Attachments in group-by) but it seems to work fine as is
     groupbyColumns,
     widgetType: pageWidget.type,
     ancestors: new Set(),
@@ -284,7 +296,7 @@ function fromColumns(table: TableRec, mainNode: LinkNode, tableExists: boolean =
     }
     const tableId = getReferencedTableId(column.type.peek());
     if (tableId) {
-      nodes.push({...mainNode, tableId, column});
+      nodes.push({...mainNode, tableId, column, isAttachments: column.type.peek() == "Attachments"});
     }
   }
   return nodes;