Minesweeper: Use a faster method to generate game field

The existing method was simply using a "randomly generate until it fits
our criteria" method to generate a game field. While this worked OK in
most cases, the run time was increasing seriously in boards whose
mine count / board size ratio was too big.

The new approach simply generates every possible mine location, shuffles
the array and picks its head. This uses more memory (shouldn't be a big
deal since minesweeper boards are generally miniscule) but runs much
quicker. The generation could still use some improvement (regarding
error handling), though :^)

Userland/Games/Minesweeper/Field.h

@@ -105,6 +105,7 @@ public:
     void set_single_chording(bool new_val);
 
     void reset();
+    void generate_field(size_t start_row, size_t start_column);
 
 private:
     Field(GUI::Label& flag_label, GUI::Label& time_label, GUI::Button& face_button, Function<void(Gfx::IntSize)> on_size_changed);