Implement "form" file picker

[bdach]

Continuation of #29643.

2024-09-26.14-40-20.mp4

[peppy]

This is a bit annoying isn't it:

Not really sure how to fix though. Seems that border is applied using different floating point values to masking.

[smoogipoo]

This is a bit annoying isn't it:

Well, I do have this: https://github.com/smoogipoo/osu-framework/tree/shader-scissor

Maybe it'll happen at some point, but the above branch is not enough because

• the border is being drawn by an overlay with MaskingSmoothness = 1 so it has alpha on the outer edge,
• the content is inside a ScrollContainer with Masking = true so every masking parameter (and importantly, BorderColour) is being reset
• so the text still bleeds through a bit and doesn't re-apply the border colour itself because it doesn't see it

Kind of a messed up scenario that needs more thought.