Use PHYSFSX_gets_line_t for Current_level_name
Use PHYSFSX_gets_line_t for Current_level_palette
Use array<unique_ptr<UI_KEYPAD>> for KeyPad
Use array<char,100> for UI_KEYPAD::buttontext
Cache KeyPad[n] address
Use unique_ptr for browser
Fix memory leak in ui_messagebox
Use unique_ptr for hostage_dialog
Use unique_ptr for trigger_dialog
Use unique_ptr for centers_dialog
Use unique_ptr for robot_dialog
Use unique_ptr for wall_dialog
Remove obsolete hostage message support

This was never included in the game.  Editor support for it is
incomplete and does not allow text entry.  Remove it.
Deduce input box text length
Pass creation context to new windows
Use unique_ptr for object_dialog
Return unique_ptr from ui_add_gadget_icon
Remove unused UI_GADGET_KEYTRAP
Return unique_ptr from ui_add_gadget_userbox
Return unique_ptr from ui_add_gadget_inputbox