Updates to Keyboard Shortcuts following #1606, #1592

Because we added a bunch of new keyboard shortcuts for toggling streetview
layers, the Keyboard Shortcuts screen got pretty cluttered.  This change
includes a few things to make our keyboard shortcuts easier to work with.

- Allow localization for the new layer toggling shortcuts introduced
  in #1606, #1592 , also some code cleanup
- Move all keyboard handlers into the UI components.
  (Core systems like Map and Photos start a bit earlier and aren't really
  intended to be localized in the way the UI components are)
- Rearrange the Keyboard Shortcuts screen content so everything fits better
- Move all the localizible keys into one place in core.yaml
  Before they were all over the place.
  (I believe it's ok to move these now since per 474211d it seems
  that Translation Memory will restore them if their keys change)
- Also starting to add more "developer notes" comments to the core.yaml file.
  Comments in this file become notes that translators can see!

css/80_app.css

@@ -4911,50 +4911,46 @@ li.issue-fix-item button:not(.actionable) .fix-icon {
   }
 }
 
-.modal-shortcuts .shortcut-tab {
+.shortcut-tab {
   display: flex;
   flex-flow: row wrap;
   justify-content: space-around;
 }
 
-.modal-shortcuts .shortcut-column {
+.shortcut-column {
   width: auto;
+  height: 0;
+  margin-bottom: 20px;
 }
 
-.modal-shortcuts .shortcut-tab-tools .shortcut-column {
-  flex: 1 1 100%;
-  width: 100%;
+.shortcut-row {
 }
 
-.modal-shortcuts td {
+.shortcut-row > td {
   padding-bottom: 3px;
 }
 
-.modal-shortcuts .shortcut-section {
-  padding: 20px 0 3px 0;
+.shortcut-row > .shortcut-section {
+  padding: 15px 0 5px 0;
 }
 
-.modal-shortcuts .shortcut-keys {
+.shortcut-keys {
   padding: 0 10px;
   color: #767676;
   text-align: right;
   white-space: nowrap;
 }
-.ideditor[dir='rtl'] .modal-shortcuts .shortcut-keys {
+.ideditor[dir='rtl'] .shortcut-keys {
   text-align: left;
 }
 
-.modal-shortcuts .shortcut-keys kbd {
+.shortcut-keys kbd {
   color: #555;
+  margin: unset;
 }
-.modal-shortcuts .shortcut-keys svg {
+.shortcut-separator {
   margin: 0 3px;
 }
-.modal-shortcuts .shortcut-keys .gesture {
-  color: #333;
-  margin: 0 3px;
-}
-
 
 /* Settings Modals (custom background, custom data)
 ------------------------------------------------------- */