mate-media: update to 1.26.0

Change log:

### mate-media 1.26.0

  * Translations update
  * replace mute/unmute checkmenuitem with imagemenuitem
  * re-generate po files
  * update copyright to 2021
  * man page: mate-volume-control-applet.1 -> mate-volume-control-status-icon.1.
  * Fix desktop file validation warnings
  * remove dead function mate_panel_applet_set_background_widget

### mate-media 1.25.0

  * Translations update
  * gvc-mixer-dialogi: Refactor on_notebook_scroll_event
  * add git.mk to generate .gitignore
  * gvc-stream-applet-icon: Remove enum-conversion warning
  * build: silent build warnings for distcheck
  * build: do not override the default value for MATE_COMPILE_WARNINGS
  * panel-applet: ensure speaker can be shown alongside or above mic

(gutteridge)