userChrome: add initial version

Screenshots: https://gitlab.com/postmarketOS/pmaports/-/issues/701#note_391806241

src/userChrome/appMenu.css

@@ -0,0 +1,42 @@
+.subviewbutton,
+.toolbaritem-combined-buttons {
+  font-size: 8pt;
+}
+
+#appMenu-protection-report-button,
+#appMenuRestoreLastSession,
+#appMenu-customize-button,
+#appMenu-open-file-button,
+#appMenu-more-button,
+#appMenu-developer-button,
+#appMenu-whatsnew-button,
+#appMenu-help-button,
+#appMenu-quit-button {
+  display: none;
+}
+
+toolbarseparator {
+  display: none;
+}
+
+/* Zoom/edit: move controls to the left (so they are visible), hide label (it's obvious what they do) and spacer */
+#appMenu-zoom-controls spacer,
+#appMenu-zoom-controls label,
+#appMenu-edit-controls spacer,
+#appMenu-edit-controls label {
+  display: none;
+}
+
+/* Zoom/edit: center the controls somewhat */
+#appMenu-zoom-controls {
+  padding-left: 20px;
+}
+#appMenu-edit-controls {
+  padding-left: 15px;
+}
+
+/* Zoom: hide fullscreen button (not possible to exit fullscreen without keyboard) */
+#appMenu-fullscreen-button {
+  display: none;
+}
+

src/userChrome/root.css

@@ -0,0 +1,4 @@
+/* Reduce minimum window width */
+:root {
+  min-width: 300px !important;
+}

src/userChrome/urlbar.css

@@ -0,0 +1,49 @@
+/* Reduce minimum window width */
+#urlbar-container {
+  min-width: 150px !important;
+}
+
+/* Remove various buttons left and right of the URL bar:
+   - forward-button: also reachable via longpress of back button
+   - home-button: not important enough
+   - customizableui-special-spring: empty space
+   - library-button: also reachable via PanelUI-menu-button
+   - sidebar-button: not useful on mobile (we try to gain horizontal space)
+   - fxa-toolbar-menu-button: firefox cloud stuff, also reachable via #PanelUI-menu-button
+*/
+/* #back-button */
+#forward-button,
+/* #reload-button */
+#home-button,
+#customizableui-special-spring1,
+/* (urlbar) */
+#customizableui-special-spring2,
+#library-button,
+#sidebar-button,
+#fxa-toolbar-menu-button
+/* #PanelUI-menu-button */ {
+  display: none !important;
+}
+
+/* urlbar not focused: smaller font */
+#urlbar-input {
+  font-size: 9pt !important;
+}
+#urlbar[open] #urlbar-input {
+  font-size: 10pt !important;
+}
+
+/* Focused urlbar: hide all icons around it, so we have more space to edit the URL */
+#urlbar[focused] #identity-box,
+#urlbar[focused] #tracking-protection-icon-container,
+/* #urlbar-input */
+#urlbar[focused] #reader-mode-button,
+#urlbar[focused] #pageActionButton {
+  display: none;
+}
+
+/* Focused urlbar: use almost full screen width (let it cover buttons left and right, which are not useful when editing the URL) */
+#nav-bar[urlbar-exceeds-toolbar-bounds] #urlbar-container {
+  margin-left: -50px !important;
+  margin-right: -25px !important;
+}