Fixed brush icons loading slowly
* Changed brush icon property from an enum to a flag that toggles whether a custom file is used for the brush icon
* Changed get_brush_icon to only handle loading external icons, built-ins are handled through the regular icon system
* Modified preview icon drawing to allow built-in icons
* When not using a custom icon, a default icon is selected based on the current tool
TODO:
* Allowing preview to show built-in icons makes the brush texture selector look ugly when nothing is selected. As discussed on IRC though, the nothing-selected state needs to be clarified anyway; I'll address this in another commit
* Use image browser when selecting a custom icon
* Selecting the default icon is ugly (uses the active object's mode), this can be fixed by making brushes know which paint mode they are part of
ec19c7dffc