Renaming File menu in Explorer interface to Files for consistency with Commander interface

Historically, singular was used for consistency with Windows File Explorer. As nowadays GUI of Windows File Explorer is quite different to WinSCP, the consistency is not relevant anymore. On the other hand, inconsistency to Commander's "Files" menu caused troubles in documentation, where we needed to use "File(s)" or similar.

Source commit: 418eded22df0b27f319fc269347c7d7be2015edb

source/forms/ScpExplorer.dfm

@@ -30,8 +30,8 @@ inherited ScpExplorerForm: TScpExplorerForm
       Stretch = True
       TabOrder = 0
       object TBXSubmenuItem5: TTBXSubmenuItem
-        Caption = '&File'
-        HelpKeyword = 'ui_explorer_menu#file'
+        Caption = '&Files'
+        HelpKeyword = 'ui_explorer_menu#files'
         Hint = 'File operations'
         object TBXSubmenuItem26: TTBXSubmenuItem
           Caption = '&New'