Adapt "Simple Tab Groups" add-on (#231)

The "Simple Tab Groups" [1] add-on allows to "create, modify, and
quickly change tab groups" and is a port of one of the best Firefox
extensions that were available before "Firefox Quantum".

To adapt it the add-on icon has been added to the top bar for quick
access.

[1]: https://addons.mozilla.org/en-US/firefox/addon/simple-tab-groups

Closes GH-219

snowblocks/firefox/user.js

@@ -233,7 +233,7 @@ user_pref("browser.tabs.loadInBackground", true);
  * Available within the UI under:
  *   Customize...
  */
-user_pref("browser.uiCustomization.state", "{\"placements\":{\"widget-overflow-fixed-list\":[\"_react-devtools-browser-action\",\"ublock0_raymondhill_net-browser-action\",\"_c2c003ee-bd69-42a2-b0e9-6f34222cb046_-browser-action\",\"_a4c4eda4-fb84-4a84-b4a1-f7c1cbf2a1ad_-browser-action\"],\"PersonalToolbar\":[\"personal-bookmarks\"],\"nav-bar\":[\"back-button\",\"forward-button\",\"stop-reload-button\",\"home-button\",\"customizableui-special-spring1\",\"urlbar-container\",\"customizableui-special-spring2\",\"downloads-button\",\"_testpilot-containers-browser-action\",\"jid1-f3mymbcpz2azyl_jetpack-browser-action\"],\"toolbar-menubar\":[\"menubar-items\"],\"TabsToolbar\":[\"tabbrowser-tabs\",\"new-tab-button\",\"alltabs-button\"]},\"seen\":[\"developer-button\",\"ublock0_raymondhill_net-browser-action\",\"_react-devtools-browser-action\",\"jid1-f3mymbcpz2azyl_jetpack-browser-action\",\"webide-button\",\"_testpilot-containers-browser-action\",\"snoozetabs_mozilla_com-browser-action\",\"_a4c4eda4-fb84-4a84-b4a1-f7c1cbf2a1ad_-browser-action\", \"firefoxcolor_mozilla_com-browser-action\"],\"dirtyAreaCache\":[\"PersonalToolbar\",\"nav-bar\",\"toolbar-menubar\",\"TabsToolbar\",\"widget-overflow-fixed-list\"],\"currentVersion\":14,\"newElementCount\":10}");
+user_pref("browser.uiCustomization.state", "{\"placements\":{\"widget-overflow-fixed-list\":[\"_react-devtools-browser-action\",\"ublock0_raymondhill_net-browser-action\",\"_c2c003ee-bd69-42a2-b0e9-6f34222cb046_-browser-action\",\"_a4c4eda4-fb84-4a84-b4a1-f7c1cbf2a1ad_-browser-action\",\"jid1-om7ejgwa1u8akg_jetpack-browser-action\"],\"nav-bar\":[\"back-button\",\"forward-button\",\"stop-reload-button\",\"home-button\",\"customizableui-special-spring1\",\"urlbar-container\",\"customizableui-special-spring2\",\"downloads-button\",\"_testpilot-containers-browser-action\",\"simple-tab-groups_drive4ik-browser-action\",\"jid1-f3mymbcpz2azyl_jetpack-browser-action\",\"fxa-toolbar-menu-button\"],\"toolbar-menubar\":[\"menubar-items\"],\"TabsToolbar\":[\"tabbrowser-tabs\",\"new-tab-button\",\"alltabs-button\"],\"PersonalToolbar\":[\"personal-bookmarks\"]},\"seen\":[\"developer-button\",\"ublock0_raymondhill_net-browser-action\",\"_react-devtools-browser-action\",\"jid1-f3mymbcpz2azyl_jetpack-browser-action\",\"webide-button\",\"_testpilot-containers-browser-action\",\"snoozetabs_mozilla_com-browser-action\",\"_a4c4eda4-fb84-4a84-b4a1-f7c1cbf2a1ad_-browser-action\",\"firefoxcolor_mozilla_com-browser-action\",\"jid1-om7ejgwa1u8akg_jetpack-browser-action\",\"simple-tab-groups_drive4ik-browser-action\",\"_c2c003ee-bd69-42a2-b0e9-6f34222cb046_-browser-action\"],\"dirtyAreaCache\":[\"PersonalToolbar\",\"nav-bar\",\"toolbar-menubar\",\"TabsToolbar\",\"widget-overflow-fixed-list\"],\"currentVersion\":16,\"newElementCount\":11}");
 
 /*
  * Set the UI density to "Normal".