Use symbolic icons everywhere in the Media Library toolbar

This requires gnome-icon-theme-symbolic 3.6
mainwindow: Don't jump through hoops to call showProjectSettingsDialog
Updated Czech translation
Updated Polish translation
Updated Spanish translation
Updated Slovenian translation
mainwindow: Fix an error occurring after choosing a missing clip's new location
Fix a race between clicks on the preview widget's slider and position updates

When using the video preview widget's slider in the media library/file chooser,
the slider would often "jump" back to its previous position instead of seeking.
This is because the slider's position update timer was not inhibited while users
were interacting with the slider widget.
Use the system's default image viewer to preview images from the media library

The UX provided by dedicated image viewing apps will be better anyway.
mediafilespreviewer: Update the preview widget slider on a more frequent basis

This makes the previewer feel a bit snappier
Use symbolic icons for the "Reset to default value" dynamic widget buttons
Automatically save and restore the main window's position

This is especially useful when using detached utility windows.
Also properly set default settings values for width and height.
mainwindow: Connect signals after restoring settings
startupwizard: Ensure the welcome dialog is initially centered on its parent