Add tooltips, pointer style, and change CE class name

index.html

@@ -15,9 +15,18 @@
                 <div class="main-screen">0</div>
             </div>
             
-            <button class="clear all">C</button>
-            <button class="clear error">CE</button>
-            <button class="plus-minus">+/-</button>
+            <button class="clear all tooltip">
+                C
+                <span class="tooltiptext">Keys: c, Esc</span>
+            </button>
+            <button class="clear entry tooltip">
+                CE
+                <span class="tooltiptext">Keys: Back, Del </span>
+            </button>
+            <button class="plus-minus tooltip">
+                +/-
+                <span class="tooltiptext">Key: _</span>
+            </button>
             <button id="multiply" class="operator">*</button>