Adjust the titlebar/menubar xml and styling.

Reasoning:
- Different Linux DEs specify different titlebar button (min, max,
close) locations which we did not and could not support before.
- Previous approach copied from Firefox depended on custom offsets
specified in CSS instead of allowing the browser engine to place
elements

Changes:
- Titlebar and menubar no longer collapsed into each other with a
negative margin, instead just a single flexbox (on macOS the titlebar
is still collapsed into the tab bar)
- Support for -moz-gtk-csd-reversed-placement to correctly place the
titlebar buttons on Linux
- Removed/changed some code copied from Firefox that does not apply or
uses the wrong selectors
- Fixed the display of Linux Unity DE buttons with icons appearing outside
of button borders
- Removed the Z in the titlebar on Linux since only a few DEs use that.
- Aligned the tabbar on Linux and Windows to the left margin of the
window.

scss/components/_mainWindow.scss

@@ -58,24 +58,6 @@
 	}
 	
 	
-	
-	
-	#titlebar {
-		margin-bottom: calc(0px - var(--tab-min-height));
-		-moz-box-pack: center;
-	}
-	
-	.titlebar-buttonbox-container {
-	  -moz-box-ordinal-group: 1000;
-	}
-	
-	
-	
-	
-	
-	
-	
-	
 	/* ::::: Tabs in Titlebar :::::: */
 	
 	&:not([chromemargin]) > #titlebar {
@@ -90,10 +72,6 @@
 	  min-height: calc(var(--tab-min-height) + var(--space-above-tabbar));
 	}
 	
-	&[tabsintitlebar="true"]:not(:-moz-lwtheme) > #titlebar {
-	  -moz-appearance: -moz-window-titlebar;
-	}
-	
 	&:not([tabsintitlebar]) .titlebar-placeholder {
 	  visibility: collapse;
 	}
@@ -110,15 +88,6 @@
 		.titlebar-buttonbox {
 			-moz-appearance: -moz-window-button-box;
 		}
-		
-		/* Fullscreen and caption buttons don't move with RTL on OS X so override the automatic ordering. */
-		.titlebar-buttonbox-container:-moz-locale-dir(rtl) {
-			-moz-box-ordinal-group: 1000;
-		}
-		
-		.titlebar-buttonbox-container:-moz-locale-dir(ltr) {
-			-moz-box-ordinal-group: 0;
-		}
 
 		.titlebar-icon {
 			display: none;
@@ -131,7 +100,10 @@
 	display: none;
 }
 
-#navigator-toolbox {
+#titlebar {
+	display: flex;
+	align-items: center;
+	-moz-window-dragging: drag;
 	menu {
 		font-size: var(--zotero-font-size);
 	}