Cosmetic changes related to Dialogs (resizing/styles).

Some 'flex' objects can't be reduced to zero when resizing the dialogs. (minheight set)
In 'HostFile servers' dialog, the button's access depends on number of selected items and their location.
New styles for 'filter' dialog.

Signed-off-by: Rookie <Thierry@evo-n600.(none)>