JS: use slice() instead of splice()

slice() does the same as splice() without the unintended effect of
transforming the array.

TODO.md

@@ -45,7 +45,6 @@
 
 - Fixes
   - Newlines and quote after newline in room subtitle
-  - Remove usage of `splice()`
   - Event delegates changing height don't scroll the list
   - When selecting text and scrolling up, selection stops working after a while
     - Ensure all the text that should be copied is copied

src/qml/Chat/Composer.qml

@@ -36,7 +36,7 @@ Rectangle {
 
     readonly property int deleteCharsOnBackspace:
         lineTextUntilCursor.match(/^ +$/) ?
-        lineTextUntilCursor.match(/ {1,4}/g).splice(-1)[0].length :
+        lineTextUntilCursor.match(/ {1,4}/g).slice(-1)[0].length :
         1
 
     // property var pr: lineTextUntilCursor

src/qml/utils.js

@@ -289,6 +289,6 @@ function copyToClipboard(text) {
 
 
 function urlExtension(url) {
-    return url.toString().split("/").splice(-1)[0].split("?")[0].split(".")
-              .splice(-1)[0].toLowerCase()
+    return url.toString().split("/").slice(-1)[0].split("?")[0].split(".")
+              .slice(-1)[0].toLowerCase()
 }