Spreadsheet: Show the opened filename in the window title

Prior to this commit, there was a set_filename() function that could set
the window title, but actually it never did that.  It was called only
in one place -- by the 'Save as...' action, but it always failed to
change anything, because there was a check that tried to reassign
the same filename. :/

This patch:
1. removes that check, and therefore
2. renames the function to simply `update_window_title()`,
3. starts calling the function from more places (at startup and after
   loading a file),
4. changes the window title order
   (`{app_name} - {filename}` -> `{filename} - {app_name}`) to match
   the other applications style on the system. :^)

Userland/Applications/Spreadsheet/SpreadsheetWidget.h

@@ -27,7 +27,7 @@ public:
 
     const String& current_filename() const { return m_workbook->current_filename(); }
     Sheet* current_worksheet_if_available() { return m_selected_view ? m_selected_view->sheet_if_available() : nullptr; }
-    void set_filename(const String& filename);
+    void update_window_title();
 
     Workbook& workbook() { return *m_workbook; }
     const Workbook& workbook() const { return *m_workbook; }