Some fixes in rendering legacy plugins

Summary:
Some exploratory testing on all iOS and Android plugins, to see how they behave inside Sandy, and fixed some layout glitches (some were also present without Sandy)

General fixes:
* Introduced some niceties like searchbox resizing properly, and toolbars wrapping automatically in Sandy, rather than buttons becoming invisible
* Containers don't grow anymore by default, but take size of contents
* ScrollContainer child is now a Layout.Vertical. Layout.Vertical should be used as default container everywhere (e.g. Tabs, Panels) in the future
* Fixed layout issue if a split container had only 1 visible child
* DetailsSidebar now scrolls vertically by default
* Details sidebar would sometimes render content in-place rather than in the reserved area
* AppSelector dropdown and Plugin list will now properly ellipse (...) if there is not enough space

Plugin fixes:
* Long database / table names in Database plugin would break layout

Also fixes https://github.com/facebook/flipper/issues/1611

Reviewed By: passy

Differential Revision: D24454188

fbshipit-source-id: c60c867270900a1d4f28587d47067c6ec1072ede

desktop/app/src/reducers/connections.tsx

@@ -124,7 +124,7 @@ export type Action =
     }
   | {
       type: 'SELECT_CLIENT';
-      payload: string;
+      payload: string | null;
     }
   | RegisterPluginAction
   | {
@@ -404,7 +404,7 @@ export const starPlugin = (payload: {
   payload,
 });
 
-export const selectClient = (clientId: string): Action => ({
+export const selectClient = (clientId: string | null): Action => ({
   type: 'SELECT_CLIENT',
   payload: clientId,
 });