Restrict CMake to only C

Fix wayland renderer lagging behind by 1 keystroke

Fix vertical alignment of text.

When using the wayland renderer the entries according to the second to last keystroke where displayed while the actual selected entry matched the last keystroke.
This removes skipping the buffer change in bm_wl_window_render if window->frame_cb is not NULL.

More wayland fixes

window->height is 0 at this point anyway...

This avoids locking oneself out when running bemenu interactively,
as input would be grabbed too early without that and layer shell
does not allow to 'ungrab' focus by clicking on another window

 - reset size when it changes, so there is no blank below text
 - ignore exclude zone to overlap bar regardless of rendering order

Wayland renderer: implement bottom option

Port to wlroots layer shell

We can't handle input yet during read because renderers are very dumb.
Thus -f can potentially lock your system with grab.

reference bm_scrollbar_mode in documentation

Add C-m -> Return bind

C-m is often bound to return and it's jarring to have all the
other keybinds but not this one

Remove double slash from os anonymous file

The `bm_scrollbar_mode` constants linked to the `bm_run_result` enum in their doc comments.

the anonymous file path is created from
- the XDG_RUNTIME path (with optional slash at the end)
- a slash if the XDG_RUNTIME path didn't have one
- and the tmpfile template (which starts with a slash).
This guarantees there are always two slashes before the filename.
It does not affect the behaviour of the program, but I've removed it anyway.