(core) Add custom widget gallery

Summary:
Custom widgets are now shown in a gallery.

The gallery is automatically opened when a new custom widget is
added to a page.

Descriptions, authors, and update times are pulled from the widget
manifest.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D4309

app/client/ui2018/IconList.ts

@@ -119,6 +119,7 @@ export type IconName = "ChartArea" |
   "Public" |
   "PublicColor" |
   "PublicFilled" |
+  "Question" |
   "Redo" |
   "Remove" |
   "RemoveBig" |
@@ -280,6 +281,7 @@ export const IconList: IconName[] = ["ChartArea",
   "Public",
   "PublicColor",
   "PublicFilled",
+  "Question",
   "Redo",
   "Remove",
   "RemoveBig",

app/client/ui2018/cssVars.ts

@@ -471,6 +471,10 @@ export const theme = {
     undefined, colors.mediumGreyOpaque),
   rightPanelFieldSettingsButtonBg: new CustomProp('theme-right-panel-field-settings-button-bg',
     undefined, 'lightgrey'),
+  rightPanelCustomWidgetButtonFg: new CustomProp('theme-right-panel-custom-widget-button-fg',
+    undefined, colors.dark),
+  rightPanelCustomWidgetButtonBg: new CustomProp('theme-right-panel-custom-widget-button-bg',
+    undefined, colors.darkGrey),
 
   /* Document History */
   documentHistorySnapshotFg: new CustomProp('theme-document-history-snapshot-fg', undefined,
@@ -877,6 +881,20 @@ export const theme = {
 
   /* Numeric Spinners */
   numericSpinnerFg: new CustomProp('theme-numeric-spinner-fg', undefined, '#606060'),
+
+  /* Custom Widget Gallery */
+  widgetGalleryBorder: new CustomProp('theme-widget-gallery-border', undefined, colors.darkGrey),
+  widgetGalleryBorderSelected: new CustomProp('theme-widget-gallery-border-selected', undefined,
+    colors.lightGreen),
+  widgetGalleryShadow: new CustomProp('theme-widget-gallery-shadow', undefined, '#0000001A'),
+  widgetGalleryBgHover: new CustomProp('theme-widget-gallery-bg-hover', undefined,
+    colors.lightGrey),
+  widgetGallerySecondaryHeaderFg: new CustomProp('theme-widget-gallery-secondary-header-fg',
+    undefined, colors.light),
+  widgetGallerySecondaryHeaderBg: new CustomProp('theme-widget-gallery-secondary-header-bg',
+    undefined, colors.slate),
+  widgetGallerySecondaryHeaderBgHover: new CustomProp(
+    'theme-widget-gallery-secondary-header-bg-hover', undefined, '#7E7E85'),
 };
 
 const cssColors = values(colors).map(v => v.decl()).join('\n');