(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/TopBarCss.ts

@@ -0,0 +1,34 @@
+import {colors} from 'app/client/ui2018/cssVars';
+import {icon} from 'app/client/ui2018/icons';
+import {styled} from 'grainjs';
+
+export const cssHoverCircle = styled('div', `
+  width: 32px;
+  height: 32px;
+  background: none;
+  border-radius: 16px;
+
+  &:hover, &.weasel-popup-open {
+    background-color: ${colors.mediumGrey};
+  }
+
+  &-disabled:hover {
+    background: none;
+  }
+`);
+
+export const cssTopBarBtn = styled(icon, `
+  width: 32px;
+  height: 32px;
+  padding: 8px 8px;
+  cursor: pointer;
+  -webkit-mask-size: 16px 16px;
+  background-color: ${colors.lightGreen};
+
+  .${cssHoverCircle.className}-disabled & {
+    background-color: ${colors.darkGrey};
+    cursor: default;
+  }
+  &-slate { background-color: ${colors.slate}; }
+  &-error { background-color: ${colors.error}; }
+`);