prevent "create" button in "new document" window to be clicked multiple times

Summary:
This prevents accidental creation of multiple images when creating the image is slower than clicking (i can easily get five images open by clicking fast). for some reason, opening multiple images like this also causes an assertion to fire:

ASSERT (krita): "m_dummiesMap.contains(node)" in file /home/timo/kde4/src/krita/krita/ui/flake/kis_node_dummies_graph.cpp, line 187

With this patch it's no longer possible to create multiple new files "in parallel"

Reviewers: rempt, abrahams

Reviewed By: abrahams

Subscribers: rempt

Projects: Krita:_next

Differential Revision: https://phabricator.kde.org/D617