Before this commit, the OSD could overflow the screen when displaying
many window items. In this commit, we hide the overflowed items and
show a scrollbar to fit the OSD within the screen.
We declared `cycle_state` struct in `labwc.h` and `cycle_osd_scene`
struct in `output.h`, which was unclean in terms of separation of
concerns.
So this commit firstly moves `cycle_state` to `cycle.h`, then replaces
`cycle_osd_scene` in `output.h` with `cycle_osd_output` in `cycle.h`
which is dynamically allocated in a similar manner to
`session_lock_output`. This ensures that all states about alt-tabbing
are stored in `server->cycle`.
Also, this commit fixes a rare memory leak in `output->cycle_osd.items`
when an output is destroyed while alt-tabbing, by freeing it when the
osd tree is destroyed.
This allows changing the cycled order in the future, e.g. focused order vs
created order.
Functionally, this commit also changes the initially selected window;
before this commit, the previous/next of the topmost window was always
selected, but now the previous/next of the active window is selected first
if it is in the cycled list. This won't change behaviors for most users,
but this ensures that the user can go back to the focused window with
Alt-Tab + Alt-Shift-Tab even when it is not the topmost window.
This commit fixes the TODO in the previous commit by trying to preserve
the selected view when a view is destroyed during window cycling.
We were using the word "osd" to describe the window switcher, but it can
be used with on-screen display (OSD) disabled by
`<windowSwitcher><osd show="false">`. Let's use "cycle" instead to avoid
confusion.
