Change the sign (positive/negative) of the change amount (in pixels)
depending on whether the text direction is LTR or RTL.
Bug: T304487
Change-Id: I8f01a92e5e1094c9602d2ff334c730241193cedc
* Toolbar button to toggle a two-pane layout on and off.
* Resizable bar for the width of the two panes.
* Resizable bar for the edit box height, even when realtime preview is off.
* Only enabled when $wgWikiEditorRealtimePreview = true.
* Fires JS hooks when enabled, resized, and disabled.
Bug: T293347
Change-Id: Id09d44519249c0b7f5c33d48d524b7c92a5a9106