fx115 fixes

A number of style changes to fix layout for fx115.

- Make panes occupy all width/height based on layout.
- Display the tag splitter on windows and mac.
- Fix to odd fx115 behavior that gives an un-collapsed pane the
initial width of min-width * 2.
- Ensure that whenever there is a width attribute on contextPane, it will
be present in the style as well.
- Set width = 0 on zotero pane when it is not selected because it kept
preventing the contextPane from properly expanding.
- Context pane has its height changed on splitter drag.
- Set height of the context pane in stacked mode to 0 to avoid
having a blank gap after collapse.
- Remove negative margin before the toolbar if the collection tree
is collapsed on mac.
- Tweaks splitter styles to have mouse target of more than 1px.
Added positive z-index to make sure splitters are not covered by panes.
Splitter styles are somewhat unified for all platforms.
- Fix lookup panel sizing
- Quicksearch margin edits to not squize the input field.
- Collection and quick search fields layout fixed
- Use flex properties to fix layout
- fix outline display for editable-text
- fix the contextpane width going out of bounds
- stacked itemPane is visible after layout change
- In stacked view, prevent itemPane from being dragged so high that
it covers itemTree and overlaps with toolbar

scss/components/_mainWindow.scss

@@ -11,3 +11,6 @@
 	// Let window chrome, tabs, and toolbar shrink even when sidebars overflow
 	min-width: 0;
 }
+#browser, #zotero-pane-stack, #zotero-trees {
+	min-height: 0;
+}