Added `menu.items.padding.x` padding between item text and arrow instead.
Replaced `if (!string)` with `if (string_null_or_empty(string))` in
`font_extents()` just as a minor optimization.
ssd_part_type contains several node types that are not actually part of
server-side decorations (ROOT, MENU, OSD, etc.)
Rename it accordingly and move it to a common location, along with some
related conversion/comparison functions.
- Rename `scaled_scene_buffer` to `scaled_buffer`. This makes it clear
that `scaled_{font,img,icon}_buffers` are implementations of it.
- Move the files from `src/common` to `src/scaled-buffer` as
`scaled_icon_buffer` heavily depends on `server` and `view` etc.
For longer cases, factor out the logic to new functions.
For very short cases, just move the declaration before the switch.
v2: in one case, replace the switch with if/else.
Arrow signs are specific to submenu items, so they would be more natural
to be handled in menu.c rather than accepting "arrow" in
font_buffer_create().
Also I allowed non-positive numbers for max_width in font_buffer_create(),
in which case the natural font width is used as the buffer width.
<resize><drawContents>[yes|no] configures whether to let the clients
redraw its window content content while resizing.
When <resize><drawContents> is set to no, a multi-rect is shown to
indicate the geometry of the resized window.
