Parts dialog's up/down buttons should only be enabled if there is something to move in that direction

528018b (that commit added buttons to allow shuffeling the order of parts in a score) or later.
These Buttons are always enabled, even if there are no parts yet.
The up button should only be enabled if a part is selected that isn't already at the top of the list
The down button should only be enabled if a part is selected that isn't already at the bottom of the list

Other dialogs behave that way, like the album manager, the fluid soundfont part of the synthesizer dialog and the instruments dialog, probably the import MIDI dialog too.

Sample code can be found in ...mscore/albummanager.cpp, line 239 ff if I'm not mistaken.