Use native file dialog for Save As. Fixes crash when no default path
set.
Change Save All shortcut to C-A so it doesn't conflict w/Save As.
Fix default filename.
Fix issue #13 (file open dialog default path)
Merge pull request #1 from bistromath/guifixes

pothos-gui: Couple of minor GUI fixes.
pothos-gui: delete the old file open dialog
pothos: added simple readme with help url
Draw image of block when drag-dropping from block tree.
Correct mouseMoveEvent behavior, no functional change.
Title reflects read-only status, read-only files get the Save As dialog.
This closes bug 12.