The default `titleLayout` is updated to `icon:iconify,max,close` which
replaces the window menu button with the window icon.
When the icon file is not found or could not be loaded, the window menu
icon as before is shown.
The icon theme can be selected with `<theme><icon>`.
This commit adds libsfdo as an optional dependency. `-Dicon=disabled` can
be passsed to `meson setup` command in order to disable window icon, in
which case the window icon is always replaced with a window menu button.
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.
