Change menu position and accelerator for TOC gen.

Due to the fact, that submenu "Edit" is visible only in macOS,
let's move TOC generator to "File" menu. Also, change
accelerator to SHIFT+CTRL+T which is working without conflicts
and problems on all platforms.

lib/main-menu.js

@@ -145,6 +145,16 @@ const file = {
     {
       type: 'separator'
     },
+    {
+      label: 'Generate/Update Markdown TOC',
+      accelerator: 'Shift+Ctrl+T',
+      click () {
+        mainWindow.webContents.send('code:generate-toc')
+      }
+    },
+    {
+      type: 'separator'
+    },
     {
       label: 'Print',
       accelerator: 'CommandOrControl+P',
@@ -228,16 +238,6 @@ const edit = {
       click () {
         mainWindow.webContents.send('editor:add-tag')
       }
-    },
-    {
-      type: 'separator'
-    },
-    {
-      label: 'Generate/Update Markdown TOC',
-      accelerator: 'CommandOrControl+Alt+T',
-      click () {
-        mainWindow.webContents.send('code:generate-toc')
-      }
     }
   ]
 }