Eeschema multiple unit component nomenclature improvements (fixes 1226874).

* Replace all occurrences of the word "part" with "unit" when referring to
  multiple unit components for improved consistency.
* Replace "shared by" with "common to" when referring to objects (lines, arcs,
  pins, etc.) that are common to all units.
* Improve wording in library component properties dialog.

eeschema/viewlib_frame.cpp

@@ -261,19 +261,19 @@ LIB_VIEW_FRAME::~LIB_VIEW_FRAME()
 {
 }
 
+
 const wxChar* LIB_VIEW_FRAME::GetLibViewerFrameName()
 {
     return LIB_VIEW_FRAME_NAME;
 }
 
-/* return a reference to the current opened Library viewer
- * or NULL if no Library viewer currently opened
- */
+
 LIB_VIEW_FRAME* LIB_VIEW_FRAME::GetActiveLibraryViewer()
 {
     return (LIB_VIEW_FRAME*) wxWindow::FindWindowByName(GetLibViewerFrameName());
 }
 
+
 void LIB_VIEW_FRAME::OnCloseWindow( wxCloseEvent& Event )
 {
     SaveSettings();
@@ -509,6 +509,7 @@ void LIB_VIEW_FRAME::DClickOnCmpList( wxCommandEvent& event )
     }
 }
 
+
 void LIB_VIEW_FRAME::ExportToSchematicLibraryPart( wxCommandEvent& event )
 {
     int ii = m_CmpList->GetSelection();