Qif Import Assistant: Don't disable the whole dialog, just the Forward button

The Qif import assistant would at many steps block entirely, requiring the
user to either do whatever is necessary to move forward or to quit and
restart Gnucash. That's a bit rude, so set pages as GTK_ASSISTANT_TYPE_CONTENT
instead of GTK_ASSISTANT_TYPE_PROGRESS.

Fix the file selection page so that the forward button isn't enabled until
a file is selected.

BP

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23525 57a11ea4-9604-0410-9ed3-97b8803252fd
QifImport: Fix crash from attempting to import an empty file.

BP

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23526 57a11ea4-9604-0410-9ed3-97b8803252fd
Bug 705714 - QIF Import - File selection pop-up is not on top during qif import

Use gtk_window_set_transient_for(), not gtk_window_set_keep_on_top().
The latter forces the window on top of *everything*, at least on the
Gnome WM and on MSWindows.

BP

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23527 57a11ea4-9604-0410-9ed3-97b8803252fd