New fill tool modes

*ArmorPaint 1.0 alpha-65e7d8, Windows*

**Feature description:**
1. I'd like to be able to use Face paint mode with a brush, so that I can paint multiple faces at once, and not just one (this should be able to be mirrored)
(2. A new fill type: Submesh, which would paint a submesh of an object)
3. I also wonder if it would be possible to make a paint fill mode that paints the entire mesh that is connected to a face that is clicked on (such as when you select a mesh by "Normal" criteria in Blender edit mode)

No file or test is named. Start by locating the existing fill tool and Face paint mode, then determine how mirrored face painting currently works. Done would mean supporting multi-face painting, a Submesh fill type, and filling the connected mesh selected from a clicked face.

Written by the indexing model from the issue text.