feat(signals): support XTWINOPS 14 and 16 (and query the terminal for them on startup and SIGWINCH)

[imsnif]

This PR is part of the ongoing effort to support Sixel graphics inside Zellij.

It adds:

1. On startup and on SIGWINCH, Zellij queries the terminal for the text area size in pixels (CSI 14t) as well as the character cell size in pixels (CSI 16t).
2. Zellij uses these in order to calculate its internal cursor height/width ratio (up until now this was a static ratio which will still be the default for terminals not supporting these signals)*.
3. Zellij will also now support terminal panes querying for this info themselves (adjusted to the text area of the pane of course). The upshot here is that terminal panes will actually get a better experience than if running outside Zellij, seeing as Zellij will support both CSI 14t and CSI 16t even if the terminal it runs inside supports only one (since one can be derived from the other).

*This has a minor user facing effect when opening a new pane non-directionally (alt-n or ctrl-p + n). The place for the pane will now be more accurately calculated to reflect the largest free space on screen.