Always restore tabs when settings allow, save tabs from last window closed.

[jeremypw]

Fixes #2235.

Following feedback from other PRs, this PR leaves restoring and saving tabs to the settings only, except that only the first window restores tabs.

• Files opened from another app adds the requested tab to the active window, or opens a window with restored tabs if no active window and settings allow.
• Every window saves its tabs, if settings allow, so the tabs from the last one closed only persist.

Tabs are restored if:

• The window is the first opened window and
• The Files "restore-tabs" settings is true and
• The System "remember-history" setting is true and
• The window was not opened as administrator

Tabs are saved if:

• The Files "restore-tabs" settings is true and
• The System "remember-history" setting is true and
• The window was not opened as administrator

As a consequence of this PR it is no longer possible to open a window that does not save its tabs except by changing one of the above settings.

A further PR that exposes the "restore-tabs" setting in the AppMenu will be proposed to compensate for this.