(core) Add initial tutorials implementation

Summary:
Documents can now be flagged as tutorials, which causes them to display
Markdown-formatted slides from a special GristDocTutorial table. Tutorial
documents are forked on open, and remember the last slide a user was on.
They can be restarted too, which prepares a new fork of the tutorial.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3813

app/client/ui2018/IconList.ts

@@ -79,8 +79,10 @@ export type IconName = "ChartArea" |
   "Lock" |
   "Log" |
   "Mail" |
+  "Maximize" |
   "Memo" |
   "Message" |
+  "Minimize" |
   "Minus" |
   "MobileChat" |
   "MobileChat2" |
@@ -214,8 +216,10 @@ export const IconList: IconName[] = ["ChartArea",
   "Lock",
   "Log",
   "Mail",
+  "Maximize",
   "Memo",
   "Message",
+  "Minimize",
   "Minus",
   "MobileChat",
   "MobileChat2",

app/client/ui2018/breadcrumbs.ts

@@ -94,6 +94,7 @@ export function docBreadcrumbs(
     isDocNameReadOnly?: BindableValue<boolean>,
     isPageNameReadOnly?: BindableValue<boolean>,
     isFork: Observable<boolean>,
+    isTutorialFork: Observable<boolean>,
     isBareFork: Observable<boolean>,
     isFiddle: Observable<boolean>,
     isRecoveryMode: Observable<boolean>,
@@ -140,7 +141,7 @@ export function docBreadcrumbs(
         if (options.isSnapshot && use(options.isSnapshot)) {
           return cssTag(t("snapshot"), testId('snapshot-tag'));
         }
-        if (use(options.isFork)) {
+        if (use(options.isFork) && !use(options.isTutorialFork)) {
           return cssTag(t("unsaved"), testId('unsaved-tag'));
         }
         if (use(options.isRecoveryMode)) {

app/client/ui2018/cssVars.ts

@@ -694,6 +694,13 @@ export const theme = {
     colors.mediumGreyOpaque),
   datePickerRangeBgHover: new CustomProp('theme-date-picker-range-bg-hover', undefined,
     colors.darkGrey),
+
+  /* Tutorials */
+  tutorialsPopupBorder: new CustomProp('theme-tutorials-popup-border', undefined,
+    colors.darkGrey),
+  tutorialsPopupHeaderFg: new CustomProp('theme-tutorials-popup-header-fg', undefined,
+    colors.lightGreen),
+  tutorialsPopupBoxBg: new CustomProp('theme-tutorials-popup-box-bg', undefined, '#F5F5F5'),
 };
 
 const cssColors = values(colors).map(v => v.decl()).join('\n');