Fix locked objects modification

Fix for the issue #4297

As @doctormo says, the lock is supposed to mean "locked on the canvas" not "locked from being edited", it should be possible to change a locked item.

After understanding the flow of this issue, I found that the function "sp_desktop_set_style()" in desktop-style.cpp is responsible for applying style on selection on desktop. And it consists of the logic that prevented locked objects from being edited.

Before this fix: Before_bug_fix

After this fix: After_bug_fix