CM11 Themes: Add support for composed icons [1/2]

If an icon pack includes background, mask, and/or foreground
images and an icon is not included for a given app, an icon
will be generated using the background, mask and foreground.

Icon packs can include the following items into their
appfilter.xml
    <!-- Set deafult scale factor for the icons -->
    <scale factor="1.0" />

    <!-- Set back image for composed icons -->
    <iconback img="iconback" />

    <!-- Set the mask image for composed icons
         Original icon will be masked off using this mask -->
    <iconmask img="iconmask" />

    <!-- Set foreground image for composed icons -->
    <iconupon img="iconupon" />

Note: Mutliple iconback images can be used as follows
    <iconback img1="iconback1" img2="iconback2" img3="iconback3" />

Change-Id: I0f5fd7de4f0eae485139cbcd282c1d1e1663952f