Have the preferences window original accomodate its content

One of the labels had large text, with wrapping. If we don't set its
size policy to Minimum or MinimumExpanding, Qt does not properly set
the minimum size for it, then the dialog reduces some widgets
(typically those who can work at smaller sizes like combo boxes)

Fixes #67.
Fix creation of folder for saved searches

The rowid field of the saved search was not set to NULL when adding a
new folder. Since there was a missing field, the creation of the new
row failed and the folder was not being added.

Fixes #73.
Merge pull request #116 from guillaumecl/gui_fixes

UI fixes