add MultiChoiceSetting

This will add a new settings widget that allows to select
multiple values.

It also implements methods to define min and max count of
selectable values.

use as follows:

    <com.aokp.romcontrol.settings.MultiChoiceSetting
            android:key="romcontrol_test_setting"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:dialogTitle="@string/romcontrol_test_title"
            android:title="@string/romcontrol_test_title"
            android:entries="@array/romcontrol_test_entries"
            android:entryValues="@array/romcontrol_test_values"
            android:defaultValue="0|1"
            aokp:minSelectionCount="2"
            aokp:maxSelectionCount="4"
            />

Change-Id: I391b5b3e1da9e29ccf8abbc96fd670f448a25a98
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>