Make editor panel icons more discreet

The Godot panel icons in the main editor window is very "shouty".
In a dark theme, especially at 200% display scaling, it's a huge,
bright and shiny, near white flashlight in your eyes.

Also, not knowing what it was, I actually initially thought the icon
was missing or bugged, as it's just a white square!

editor/icons/Panels1.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#e0e0e0" d="m0 0h16v16h-16z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#e0e0e0" d="M2 0a2 2 0 0 0-2 2v12.002a2 2 0 0 0 2 2h12.002a2 2 0 0 0 2-2V2a2 2 0 0 0-2-2zm0 4h12v10H2z"/></svg>

editor/icons/Panels2.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#e0e0e0" d="m0 0v7h16v-7zm0 9v7h16v-7z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#e0e0e0" d="M2 0a2 2 0 0 0-2 2v12a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V2a2 2 0 0 0-2-2zm0 4h12v4H2zm12 6v4H2v-4z"/></svg>

editor/icons/Panels2Alt.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#e0e0e0" d="m0 0v16h7v-16zm9 0v16h7v-16z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#e0e0e0" d="M2 0a2 2 0 0 0-2 2v12a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V2a2 2 0 0 0-2-2zm0 4h5v10H2zm7 0h5v10H9z"/></svg>

editor/icons/Panels3.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#e0e0e0" d="m0 0v7h16v-7zm0 9v7h7v-7zm9 0v7h7v-7z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#e0e0e0" d="M2 0a2 2 0 0 0-2 2v12a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V2a2 2 0 0 0-2-2zm0 4h12v4H2zm0 6h5v4H2zm7 0h5v4H9z"/></svg>

editor/icons/Panels3Alt.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#e0e0e0" d="m0 0v7h7v-7zm9 0v16h7v-16zm-9 9v7h7v-7z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#e0e0e0" d="M2 0a2 2 0 0 0-2 2v12a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V2a2 2 0 0 0-2-2zm0 4h5v4H2zm7 0h5v10H9zm-7 6h5v4H2z"/></svg>

editor/icons/Panels4.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#e0e0e0" d="m0 0v7h7v-7zm9 0v7h7v-7zm-9 9v7h7v-7zm9 0v7h7v-7z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#e0e0e0" d="M2 0a2 2 0 0 0-2 2v12a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V2a2 2 0 0 0-2-2zm0 4h5v4H2zm7 0h5v4H9zm-7 6h5v4H2zm7 0h5v4H9z"/></svg>