Log Message:
mate-panel: update to 1.26.3

Change log:

### mate-panel 1.26.3

  * update translations
    windows-list.ui: lower gtk3 required version
    panel-applet: Fix leak in no-background code path
    clock: Fix memory leak
    wncklet: Fix crash thumbnailing dying or otherwise inaccessible windows
    notification_area: Reduce lifetime of the settings local variable (#1309)
    Register SNI host only if SNI is enabled
    wncklet: do not exceed workspace number when setting n_rows