Spell browser lost its header

[PowerWyrm]

Due to this commit:

https://github.com/angband/angband/commit/fac5a0056f5402900330eaf588423140ad9913a3

In menu_refresh(), the menu header is now displayed before browse hook is called. With the "browse spell" menu, here's what happens:
- browsing is initiated by spell_menu_browse()
- screen_save() is called, which makes a copy of the display
- menu_select() is called
- menu_refresh() is called
- menu header is displayed
- browser hook is called: spell_menu_browser()
- screen_load() is called, which reloads the saved copy of the display... without the menu header!
- spell description is displayed
- spell list is displayed (spell_menu_display)

spell_menu_browser() should probably only partially erase the display...