Add hover states and basic context menu to new room list

The 'sort by' radio buttons are blocked by https://github.com/matrix-org/matrix-react-sdk/pull/4731 as it contains the styles needed.

The 'unread rooms' checkbox is intentionally not hooked up. This is a more complicated refactoring that needs to be done.

The message preview checkbox works, though the previews remain hardcoded in this change. The primary intent of this change is to have a good enough context menu and the hover states.

The hover states are as described in the design.

src/stores/room-list/ListLayout.ts

@@ -20,10 +20,12 @@ const TILE_HEIGHT_PX = 44;
 
 interface ISerializedListLayout {
     numTiles: number;
+    showPreviews: boolean;
 }
 
 export class ListLayout {
     private _n = 0;
+    private _previews = false;
 
     constructor(public readonly tagId: TagID) {
         const serialized = localStorage.getItem(this.key);
@@ -31,9 +33,19 @@ export class ListLayout {
             // We don't use the setters as they cause writes.
             const parsed = <ISerializedListLayout>JSON.parse(serialized);
             this._n = parsed.numTiles;
+            this._previews = parsed.showPreviews;
         }
     }
 
+    public get showPreviews(): boolean {
+        return this._previews;
+    }
+
+    public set showPreviews(v: boolean) {
+        this._previews = v;
+        this.save();
+    }
+
     public get tileHeight(): number {
         return TILE_HEIGHT_PX;
     }
@@ -48,7 +60,7 @@ export class ListLayout {
 
     public set visibleTiles(v: number) {
         this._n = v;
-        localStorage.setItem(this.key, JSON.stringify(this.serialize()));
+        this.save();
     }
 
     public get minVisibleTiles(): number {
@@ -63,9 +75,14 @@ export class ListLayout {
         return px / this.tileHeight;
     }
 
+    private save() {
+        localStorage.setItem(this.key, JSON.stringify(this.serialize()));
+    }
+
     private serialize(): ISerializedListLayout {
         return {
             numTiles: this.visibleTiles,
+            showPreviews: this.showPreviews,
         };
     }
 }