Bugfix: Honor preferences when opening links via doubleclick or spacebar.

When this fix is applied, the default target for opened links (internal
or external browser) is taken from the preferences.

Fixes #1056, #1089, #1098
Extract common code for opening a link

Previously, the following functions used almost the same code:
- on_Itemlist_row_activated
- on_popup_launch_item_selected
- on_popup_launch_item_in_tab_selected
- on_popup_launch_item_external_selected

This common code was extracted to a new function.
Merge pull request #6 from tiwoc/refactor-launch

 Extract common code for opening a link