Add a BUF_INIT macro, which makes it easier to initialize a struct buf
to an empty string (without a heap allocation).
Add buf_move() to move the contents of one struct buf to another (the
source is reset to BUF_INIT, analogous to C++ move-assignment).
Use buf_reset() instead of directly calling `free(s->buf)` since the
internal buf may not always be allocated by malloc() now.
Some themes don't have hover variants for button pixmaps.
It looks better visually to use the non-hover variants as fallbacks
rather than the built-in 6x6 pixmaps.
