(core) Implement UI for trigger formulas.

Summary:
- Implement UI with "Apply to new records" and "Apply on record changes"
  checkboxes, and options for selecting which changes to recalculate on.
- For consistency, always represent empty RefList as None
- Fix up generated SchemaTypes to remember that values are encoded.

Included test cases for the main planned use cases:
- Auto-filled UUID column
- Data cleaning
- NOW() formula for record's last-updated timestamp.
- Updates that depend on other columns.

Test Plan: Added a browser test.

Reviewers: jarek

Reviewed By: jarek

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D2885

app/client/ui2018/select.ts

@@ -0,0 +1,48 @@
+import {colors, vars} from 'app/client/ui2018/cssVars';
+import {styled} from 'grainjs';
+
+// Import popweasel so that the styles we define here are included later in CSS, and take priority
+// over popweasel styles, when used together.
+import 'popweasel';
+
+/**
+ * Style for a select dropdown button.
+ *
+ * This incorporates styling from popweasel's select, so that it can be used to style buttons that
+ * don't use it.
+ */
+export const cssSelectBtn = styled('div', `
+  position: relative;
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  width: 100%;
+  height: 30px;
+  line-height: 16px;
+  background-color: white;
+  color: ${colors.dark};
+  --icon-color: ${colors.dark};
+  font-size: ${vars.mediumFontSize};
+  padding: 5px;
+  border: 1px solid ${colors.darkGrey};
+  border-radius: 3px;
+  cursor: pointer;
+  overflow: hidden;
+  white-space: nowrap;
+  text-overflow: ellipsis;
+  -webkit-appearance: none;
+  -moz-appearance: none;
+  user-select: none;
+  -moz-user-select: none;
+  outline: none;
+
+  &:focus {
+    outline: none;
+    box-shadow: 0px 0px 2px 2px #5E9ED6;
+  }
+
+  &.disabled {
+    color: grey;
+    cursor: pointer;
+  }
+`);