(core) Mappings improvements

Summary:
- Adding new icon for calendar view (the old one by just bigger)
- When there are no columns to map the select box is grayed out
- Optional mappings can be cleared now

Test Plan: Added

Reviewers: JakubSerafin

Reviewed By: JakubSerafin

Subscribers: JakubSerafin

Differential Revision: https://phab.getgrist.com/D4066

static/ui-icons/Data Types/TypeCalendar.svg

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   width="16px"
+   height="16px"
+   viewBox="0 0 16 16"
+   version="1.1">
+  <g
+     stroke="none"
+     stroke-width="1"
+     fill="none"
+     fill-rule="evenodd">
+    <path
+       d="M 5.1438183,3.2089992 H 11.143818 V 0.79814978 c 0,-0.29037145 0.243675,-0.49817134 0.5,-0.5 0.256325,-0.001829 0.5,0.21305073 0.5,0.5 V 3.2089992 h 2.129941 c 0.828427,0 1.5,0.6715729 1.5,1.5 v 9.5887298 c 0,0.828427 -0.671573,1.5 -1.5,1.5 H 1.8700589 c -0.8284271,0 -1.49999996,-0.671573 -1.49999996,-1.5 V 4.7089992 c 0,-0.8284271 0.67157286,-1.5 1.49999996,-1.5 H 4.1438183 V 0.79814978 c 0,-0.27614238 0.2044615,-0.50115994 0.5,-0.5 0.2955385,0.00116 0.5,0.2230347 0.5,0.5 z M 1.3700589,7.1198486 H 14.773759 V 4.7089992 c 0,-0.2761424 -0.223858,-0.5 -0.5,-0.5 H 1.8700589 c -0.2761423,0 -0.5,0.2238576 -0.5,0.5 z m 13.4037001,1 H 1.3700589 v 6.1778804 c 0,0.276142 0.2238577,0.5 0.5,0.5 H 14.273759 c 0.276142,0 0.5,-0.223858 0.5,-0.5 z"
+       id="Combined-Shape"
+       style="fill:#000000;fill-rule:nonzero"/>
+  </g>
+</svg>