(core) move client code to core

Summary:
This moves all client code to core, and makes minimal fix-ups to
get grist and grist-core to compile correctly.  The client works
in core, but I'm leaving clean-up around the build and bundles to
follow-up.

Test Plan: existing tests pass; server-dev bundle looks sane

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2627

app/client/ui/AddNewButton.ts

@@ -0,0 +1,75 @@
+import {colors, vars} from 'app/client/ui2018/cssVars';
+import {icon} from 'app/client/ui2018/icons';
+import {dom, DomElementArg, Observable, styled} from "grainjs";
+
+export function addNewButton(isOpen: Observable<boolean> | boolean = true, ...args: DomElementArg[]) {
+  return cssAddNewButton(
+    cssAddNewButton.cls('-open', isOpen),
+    // Setting spacing as flex items allows them to shrink faster when there isn't enough space.
+    cssLeftMargin(),
+    cssAddText('Add New'),
+    dom('div', {style: 'flex: 1 1 16px'}),
+    cssPlusButton(cssPlusIcon('Plus')),
+    dom('div', {style: 'flex: 0 1 16px'}),
+    ...args,
+  );
+}
+
+export const cssAddNewButton = styled('div', `
+  display: flex;
+  align-items: center;
+  margin: 22px 0px 22px 0px;
+  height: 40px;
+  color: ${colors.light};
+  border: none;
+  border-radius: 4px;
+
+  cursor: default;
+  text-align: left;
+  font-size: ${vars.bigControlFontSize};
+  font-weight: bold;
+  overflow: hidden;
+
+  --circle-color: ${colors.lightGreen};
+
+  &:hover, &.weasel-popup-open {
+    --circle-color: ${colors.darkGreen};
+  }
+  &-open {
+    margin: 22px 16px 22px 16px;
+    background-color: ${colors.lightGreen};
+    --circle-color: ${colors.darkGreen};
+  }
+  &-open:hover, &-open.weasel-popup-open {
+    background-color: ${colors.darkGreen};
+    --circle-color: ${colors.darkerGreen};
+  }
+`);
+const cssLeftMargin = styled('div', `
+  flex: 0 1 24px;
+  display: none;
+  .${cssAddNewButton.className}-open & {
+    display: block;
+  }
+`);
+const cssAddText = styled('div', `
+  flex: 0 0.5 content;
+  white-space: nowrap;
+  min-width: 0px;
+  display: none;
+  .${cssAddNewButton.className}-open & {
+    display: block;
+  }
+`);
+const cssPlusButton = styled('div', `
+  flex: none;
+  height: 28px;
+  width: 28px;
+  border-radius: 14px;
+  background-color: var(--circle-color);
+  text-align: center;
+`);
+const cssPlusIcon = styled(icon, `
+  background-color: ${colors.light};
+  margin-top: 6px;
+`);