SharePoint 2013: addanapp page only shows 3 buttons [addanapp page is broken]

It is a bug in the designer manager, you experience this bug if you use the designer manager to create a master page.
The image below shows the correct addanapp page. If the bug happens, you will not be able to see the buttons in the red circle.

To fix this, add the following code to your custom.master file.

  <SharePoint:AjaxDelta id="DeltaPlaceHolderPageTitleInTitleArea" runat="server">  
  <asp:ContentPlaceHolder id="PlaceHolderPageTitleInTitleArea" runat="server">  
  <SharePoint:SPTitleBreadcrumb  
    runat="server"  
    RenderCurrentNodeAsLink="true"  
    SiteMapProvider="SPContentMapProvider"  
    CentralAdminSiteMapProvider="SPXmlAdminContentMapProvider">  
  <PATHSEPARATORTEMPLATE>  
   <SharePoint:ClusteredDirectionalSeparatorArrow runat="server" />  
  </PATHSEPARATORTEMPLATE>  
   </SharePoint:SPTitleBreadcrumb>  
  </asp:ContentPlaceHolder>  
  </SharePoint:AjaxDelta>  
  <SharePoint:AjaxDelta BlockElement="true" id="DeltaPlaceHolderPageDescription" CssClass="ms-displayInlineBlock ms-normalWrap" runat="server">  
  <a href="javascript:;" id="ms-pageDescriptionDiv" style="display:none;">  
   <span id="ms-pageDescriptionImage">&#160;</span>  
  </a>  
  <span class="ms-accessible" id="ms-pageDescription">  
   <asp:ContentPlaceHolder id="PlaceHolderPageDescription" runat="server" />  
  </span>  
  <SharePoint:ScriptBlock runat="server">  
   _spBodyOnLoadFunctionNames.push("setupPageDescriptionCallout");  
  </SharePoint:ScriptBlock>  
  </SharePoint:AjaxDelta>  

I added it inside the pageTitle
<h1 id="pageTitle" class="ms-core-pageTitle"> I added it here </h1>

SharePoint 2013: Where is my dll?

Starting from .NET 4.0, there is a new ‘GAC’ folder “c:\windows\microsoft.net\assembly“.