Add align_x and align_y helpers to Scrollable

[hecrj]

These helpers can come in handy when you only want to change the alignment of a Scrollable without changing its direction.

[alex-ds13]

@hecrj Wouldn't it make more sense for this functions to take an alignment::Horizontal and alignment::Vertical like the ones from the container widget? To keep it consistent?

[hecrj]

@alex-ds13 That wouldn't work because those enums have 3 variants, while scrollable::Alignment has only 2.

[alex-ds13]

@alex-ds13 That wouldn't work because those enums have 3 variants, while scrollable::Alignment has only 2.

Oh I didn't notice this Alignment was specific for the scrollable... I thought it was the global Alignment.

Couldn't you use the version with 3 variants anyway and simply ignore the variant that doesn't make sense? It's just that this many different Alignments can become confusing...