Default RGBA brushtips to Lightness mode with Auto-adjust

Changes default brush mode of RGBA brushtips from Color Image to Lightness Map, and changes default setting of Auto-adjust Neutral Point checkbox to checked. Also makes it so brushtip settings are kept throughout a Krita session, and the Brush Mode setting is included in the "Preserve Brush Preset Settings" option.

See discussion at https://krita-artists.org/t/change-the-default-settings-for-rgba-brushtips/25251 for context.

Edited: no longer marked WIP, because I was able to recreate the crash in Krita nightly version, so it's not related to this patch. I'll create a separate MR when I figure out how to fix the crash issue.