[import] Fix leaking AbstractFileFilters

Summary:
Fix memory leaks due to recreation of AbstractFileFilter for each call
of ImportFileWidget::currentFileFilter(). The filter now is cached
as a member of ImportFileWidget and reused if the file type does not change.
A new method AbstractFileFilter::type() returns the type of the
current filter.

Test Plan:

• import different files
• output (during debugging) the number of newly created filter instances

Reviewers: LabPlot, asemke

Reviewed By: LabPlot, asemke

Subscribers: asemke, kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D16663