Unify cancel button & escape keybind for pages

AddChat and AccountSettings:

- When clicking the cancel button, if there's no previous back to go
  back to, focus the main pane instead of doing nothing

AccountSettings:

- Always enable the cancel button, not just when the form has been
  modified

AddChat, AccountSettings, ServerBrowser:

- Pressing escape will do the same thing as clicking the cancel button

src/gui/Pages/AddAccount/ServerBrowser.qml

@@ -24,6 +24,9 @@ HBox {
     property string connectFutureId: ""
     property string fetchServersFutureId: ""
 
+    readonly property bool canFocusMainPane:
+        mainUI.mainPane.normalOrForceCollapse && mainUI.accountsPresent
+
     signal accepted()
 
     function takeFocus() { serverField.item.field.forceActiveFocus() }
@@ -162,11 +165,7 @@ HBox {
                 visible: Layout.preferredWidth > 0
 
                 Layout.fillHeight: true
-                Layout.preferredWidth:
-                    mainUI.mainPane.normalOrForceCollapse &&
-                    mainUI.accountsPresent ?
-                    implicitWidth :
-                    0
+                Layout.preferredWidth: box.canFocusMainPane ? implicitWidth : 0
 
                 Behavior on Layout.preferredWidth { HNumberAnimation {} }
             }
@@ -221,6 +220,8 @@ HBox {
     onKeyboardAccept:
         if (serverField.item.apply.enabled) serverField.item.apply.clicked()
 
+    onKeyboardCancel: if (box.canFocusMainPane) mainUI.mainPane.toggleFocus()
+
     onAccepted: window.saveState(this)
 
     Component.onDestruction: