ui: add GroupFilter form field(container)

ui: add GroupFilter form field(container)

this contains a grid + button + hidden field which lets the user
add group filters one by one. the first column is the type selector
(type, group, regex) and the second column shows the relevant
input field (groupselector, kvcombobox for type, and textfield for regex)

i had to hack a little to get access to the widgets of the
fieldcontainer, since we cannot simply access the widget of a column
from another column (which we need to show the correct one when changing
the type), also we cannot traverse the widget hirachy in the usual way,
since extjs seems to build it differently for widgetcolumns.

to solve this, i added references of the widgets to the record, and a
reference of the record to the widgets. since this is now a cyclic
reference, i solve that in 'removeFilter' and in 'beforedestroy' of the grid
by removing the references again

also contains a small css style to remove the padding in the rows

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>