Remove -Wmissing-field-initializers warnings

[rbuj]

--
pluma-ui.h:47:2: warning: missing initializer for field 'accelerator' of 'GtkActionEntry' {aka 'const struct _GtkActionEntry'} [-Wmissing-field-initializers]
   47 |  { "File", NULL, N_("_File") },
      |  ^
--
pluma-ui.h:48:2: warning: missing initializer for field 'accelerator' of 'GtkActionEntry' {aka 'const struct _GtkActionEntry'} [-Wmissing-field-initializers]
   48 |  { "Edit", NULL, N_("_Edit") },
      |  ^
--
pluma-ui.h:49:2: warning: missing initializer for field 'accelerator' of 'GtkActionEntry' {aka 'const struct _GtkActionEntry'} [-Wmissing-field-initializers]
   49 |  { "View", NULL, N_("_View") },
      |  ^
--
pluma-ui.h:50:2: warning: missing initializer for field 'accelerator' of 'GtkActionEntry' {aka 'const struct _GtkActionEntry'} [-Wmissing-field-initializers]
   50 |  { "Search", NULL, N_("_Search") },
      |  ^
--
pluma-ui.h:51:2: warning: missing initializer for field 'accelerator' of 'GtkActionEntry' {aka 'const struct _GtkActionEntry'} [-Wmissing-field-initializers]
   51 |  { "Tools", NULL, N_("_Tools") },
      |  ^
--
pluma-ui.h:52:2: warning: missing initializer for field 'accelerator' of 'GtkActionEntry' {aka 'const struct _GtkActionEntry'} [-Wmissing-field-initializers]
   52 |  { "Documents", NULL, N_("_Documents") },
      |  ^
--
pluma-ui.h:53:2: warning: missing initializer for field 'accelerator' of 'GtkActionEntry' {aka 'const struct _GtkActionEntry'} [-Wmissing-field-initializers]
   53 |  { "Help", NULL, N_("_Help") },
      |  ^
--
pluma-ui.h:108:2: warning: missing initializer for field 'accelerator' of 'GtkActionEntry' {aka 'const struct _GtkActionEntry'} [-Wmissing-field-initializers]
  108 |  { "ViewHighlightMode", NULL, N_("_Highlight Mode") },
      |  ^
--
pluma-file-browser-widget.c:792:2: warning: missing initializer for field 'accelerator' of 'GtkActionEntry' {aka 'struct _GtkActionEntry'} [-Wmissing-field-initializers]
  792 |  {"FilterMenuAction", NULL, N_("_Filter")}
      |  ^