Allow external entity definition files to have a relative path.

Signed-off-by: Kristian Duske <kristian.duske@gmail.com>
Add tooltip to entity browser.

Signed-off-by: Kristian Duske <kristian.duske@gmail.com>
Show path selection dialog when adding an external entity definition file.

Signed-off-by: Kristian Duske <kristian.duske@gmail.com>