Qmake: Fix Build File (context) menu for non-source files

Enable Build File for source or header files only and
de-duplicate the corresponding code.

Before, the menus were incorrectly enabled for e.g. .pro
and .ui files also: While the menu was already hidden for
these  non-source files in updateContextActions(), the
following  call to updateBuildFileAction() did enable it
again.

Task-number: QTCREATORBUG-19098
Change-Id: I87f40e64ab044b33be9c465d1c5dfa36c3cf1260
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/qmakeprojectmanager/qmakeprojectmanagerplugin.h

@@ -36,7 +36,10 @@ namespace ProjectExplorer {
 class Project;
 class Target;
 }
-namespace Utils { class ParameterAction; }
+namespace Utils {
+class FileName;
+class ParameterAction;
+}
 
 namespace QmakeProjectManager {
 
@@ -70,6 +73,8 @@ private:
     void updateContextActions();
     void buildStateChanged(ProjectExplorer::Project *pro);
     void updateBuildFileAction();
+    void disableBuildFileMenus();
+    void enableBuildFileMenus(const Utils::FileName &file);
 
     QmakeManager *m_qmakeProjectManager = nullptr;
     QmakeProject *m_previousStartupProject = nullptr;